Anita Rogowicz-Frontczak M.D., Ph.D. Department of Internal Medicine and Diabetology, Poznan University of Medical Science, Mickiewicza 2, 60–834 Poznan, phone: +48 608 597 343, e-mail: anitrog@gmail.com

Introduction

Most hormones, through their antagonistic action to insulin, lead to increased hepatic glucose production and reduced utilisation at the peripheral tissues. If they are secreted in excess, or are unbalanced in relation to the level of insulin, they can lead to various degrees of disorders of glucose metabolism. Dysregularities in hormonal metabolism usually lead to compensatory hyperinsulinaemia in response to increasing insulin resistance. Some hormones, by acting on pancreatic β cells, stimulate or reduce the secretion of insulin. Hormones of potential diabetogenic effect include: growth hormone (GH), glucocorticosteroids (GS), thyroxine, catecholamines, aldosterone, parathyroid hormone, glucagon, and somatostatin. Changes in the sensitivity of cells to insulin action occur in the course of the majority of endocrine disorders. For this reason, patients with endocrinopathies should also be evaluated for carbohydrate metabolism disorders [1].

Insulin resistance and methods of treatment

Insulin resistance is defined as a disorder of glucose homeostasis involving reduced sensitivity of muscles, adipose tissue, liver, and other tissues to the action of insulin, despite its normal or elevated level in the blood. Insulin resistance may be accompanied by various disorders, such as: impaired glucose tolerance, diabetes, hypercholesterolaemia, hypertriglyceridaemia, obesity, and hypertension. Insulin acts through specific receptors present on the surface of most cells in the body. The highest presence of these receptors was found on fat cells, hepatocytes, and cells of striated muscles. Insulin resistance may be also due to the presence of hormones antagonistic to insulin (e.g. cortisol, glucagon, thyroid hormones) [2].

In the course of hormonal imbalance the best therapeutic effects in improving the sensitivity of tissues to insulin action are achieved with their effective treatment. However, complete recovery of certain endocrine diseases is not always possible. The normalisation of glucose metabolism in the course of endocrinopathy depends on many factors, such as: age, duration and severity of hormonal or metabolic disorders, and genetic predisposition [3]. Therefore, to obtain a reduction of insulin resistance, lifestyle changes and pharmacological treatment should be introduced.

When impaired insulin sensitivity is a consequence of obesity, lifestyle changes aimed at reducing body weight through diet and exercise lead to a reduction of insulin resistance.

Metformin is a first-line drug in the treatment of type 2 diabetes, but also in pre-diabetes and impaired sensitivity to insulin. Metformin reduces insulin resistance in the liver and peripheral tissues (muscles and adipose tissue). The mechanism involves tyrosine kinase activity associated with insulin receptors, stimulation of translocation of glucose transporter GLUT-4 to cell membrane, glycogen synthesis in hepatocytes, and the activation of AMP-activated kinase (AMPK) [4].

Another group of drugs improving peripheral insulin sensitivity are glitazones. They are PPAR-γ agonists, with highest expression demonstrated in adipose tissue, so their action at the adipocyte is of particular importance in the regulation of insulin sensitivity. They also affect the uptake and storage of free fatty acids (FFA) in adipocytes, and thus lead to a decrease in their plasma concentration. Glitazones protect other tissues like the liver, skeletal muscles, and pancreatic β cells, from excessive accumulation of fat. They stimulate the signalling pathway for insulin, increase glucose transport into cells and glycogen synthesis, and improve the function of mitochondria. They also show a protective effect on pancreatic islet β cells. Unfortunately, through the influence on the proliferation of adipocytes, glitazone therapy is accompanied by an increase in the accumulation of subcutaneous fat. They also cause peripheral oedema, resulting in weight gain and increased heart failure. Currently, due to the cardiovascular safety of the therapy, only pioglitazone may be used [5].

Drugs acting on the axis of incretin

The incretin effect is a phenomenon of action of the enteropancreatic axis, where the intake of food and the consequent rise in glucose levels leads to secretion of gastrointestinal hormones, mainly glucagon-like peptide 1 (GLP-1). GLP-1 stimulates insulin secretion in response to the glycaemic stimulus, inhibits glucagon secretion, slows gastric motility, and is responsible for the feeling of satiety via the nervous system. It has been shown that in obese and type 2 diabetic patients the incretin effect is impaired, and the concentration of GLP-1 in serum is lower in relation to healthy subjects. Because of the above facts, recently in the treatment of type 2 diabetes, prediabetes, and obesity, GLP-1 analogues have been introduced with good effect [6]. Gliptins are another group of incretin-based therapies. Gliptins inhibit the enzyme dipeptidyl peptidase-4 (DPP-4), which inactivates GLP-1. This results in an increase in the concentration of GLP-1, and the prolongation of the action of this gut hormone. However, they do not cause inhibition of gastric emptying and less often have an impact on the reduction of body weight. The advantage is the possibility of administering them orally, in contrast to GLP-1 analogues, which are used for medical treatment in the form of injection [6].

Inhibitors of sodium-glucose cotransporter (SGLT2) – flosins – are a relatively new class of drugs, independent of insulin, acting by lowering the threshold glucose reabsorption in the kidney. Through their effects on glucosuria, they increase the loss of calories and have the effect of reducing body weight and reducing blood pressure. They also decrease glucose toxicity and postprandial blood glucose, and have a beneficial effect on the components of the metabolic syndrome, thereby also contributing to the improvement of insulin sensitivity [7].

Insulin resistance in endocrinopathies – treatment options

Acromegaly

Acromegaly is characterised by excessive secretion of growth hormone (GH) [8]. Carbohydrate disorders often accompany the above pathology. Diabetes mellitus occurs in approximately 13–30% and impaired glucose tolerance in 60–70% of patients [9, 10]. The degree of disorder of carbohydrate metabolism is greater in the active form of the disease and correlates with the duration of acromegaly, age, concentration of GH, insulin-like growth factor-1 (IGF-1), and IGF-binding protein (IGFBP- 3, insulin-like growth factor-3-binding protein) [10]. The pathomechanism of insulin resistance in this disease is complex. Long-term supraphysiological GH concentration interferes with both insulin action in the liver and the other peripheral tissues. This leads to enhanced production of glucose by the liver and decreased utilisation of glucose in peripheral tissues. This is probably due to disturbances in the production and action of a second messenger in the insulin receptor. In addition, growth hormone increases lipolysis of adipose tissue, and increased concentration and oxidation of fatty acids enhances insulin resistance [11].

Treatment of disturbed of carbohydrate metabolism in acromegaly is aimed at reducing insulin resistance. A significant proportion of patients with overt diabetes (up to 30%) require the introduction of insulin. Effective acromegaly treatment by adenomectomy results in improved glucose tolerance, decreasing insulin resistance and hyperinsulinism. However, in the case of overt diabetes the effect of remission of active acromegaly of recovery of carbohydrate metabolism disorder is difficult to predict [12, 13]. Somatostatin analogues, like octreotide and lanreotide, used in both in the preoperative procedure and after non-radical resection of pituitary adenoma, are characterised by complex hormonal activity. They result in the reduction in endogenous GH responsible for the diabetogenic effect of hyperglycaemia. In another way they decrease the secretion of insulin, IGF-1, and IGF-BP1 growth [14]. Treatment with octreotide may improve the glycaemic control in patients with diabetes or impaired glucose tolerance. However, it can aggravate or cause hyperglycaemia in patients with minimal metabolic disorders or no disturbance in the carbohydrate metabolism [15]. Recently, a new somatostatin analogue, pasireotide, acting through many types of somatostatin receptors, was introduced. Although the results of treatment in control of acromegaly look promising, the observed adverse effect on carbohydrate metabolism is an important limiting factor. It stems from a reduction in insulin secretion and weakening of the incretin effect [16]. Incretins seem to be most effective in the control of hyperglycaemia during pasireotide therapy [17]. Instead, it has been shown that the use of the GH receptor antagonist pegvisomant in the treatment of acromegaly improves glycaemia and reduces the percentage of glycated haemoglobin (HbA1c) in diabetic patients [8].

Hyperprolactinaemia

Due to the structural similarity of prolactin to the growth hormone in the course of hyperprolactinaemia, insulin resistance and carbohydrate disorders may occur. A possible mechanism suggested is a down-regulation of insulin receptors and increased concentration of FFAs [18]. Hypogonadism and the anti-dopaminergic effect of hyperprolactinaemia also promote weight gain and the occurrence of the atherogenic lipid profile [19]. Effective treatment with dopamine agonists restores normal prolactin levels and normal sensitivity of peripheral tissues to insulin. However, the superior role of cabergoline above bromocriptine has been shown in this effect. Six-month treatment of pituitary adenoma secreting prolactin with both cabergoline and bromocriptine normalised prolactin levels, but for cabergoline also beneficial effects in reducing insulin resistance index (HOMA-IR), blood levels of triglycerides, postprandial glucose, IGF-1, FFA, chronic inflammation markers like C-reactive protein, fibrinogen, and homocysteine were found [20]

Hypercortisolism

An excess of glucocorticosteroids (GS), irrespective of their source (endogenous or iatrogenic) leads to abdominal obesity, increased accumulation of body fat, higher blood pressure, lipid disorders, insulin resistance, glucose intolerance, or diabetes [21]. The most common reason for endogenous hypercortisolism is a pituitary adenoma secreting corticotropin hormone (ACTH) [22].

Glucocorticoids affect the activation of the liver enzyme phosphoenolpyruvate carboxykinase, increasing proteolysis in skeletal muscle and lipolysis in adipose tissue, which ultimately provides more substrate for gluconeogenesis. Enhanced lipolysis in adipose tissue results in an increase in circulating FFAs and affects the development of the phenomenon of reduced insulin sensitivity [23]. Insulin resistance induced by hypercortisolaemia probably has a post-receptor nature. Cortisol also affects the increase of leptin production by fat cells; therefore, patients with Cushing’s syndrome (CS) have leptin resistance similar to those with obesity and type 2 diabetes [24]. GS also reduces expression of the adiponectin gene, which reflects good insulin sensitivity. GS also affect reduction of brown adipose tissue in favour of the metabolically disadvantaged white adipose tissue. It is also known that GS has a central effect on the nucleus of the hypothalamus and enhances secretion of the neuropeptide Y (NPY), which is associated with increased appetite, weight gain, and hyperinsulinaemia [25].

The normalisation of cortisol in patients with Cushing’s syndrome leads to improved body composition through a visible decline in the content of visceral adipose tissue [21]. However, metabolic effects expressed by reduced cardiovascular risk depend on the time of exposure to hypercortisolism [26].

Disorders of carbohydrate metabolism in Cushing’s syndrome are very common. 40–45% of patients develop diabetes, and 10-30% have impaired glucose tolerance. The degree of hyperglycaemia correlates with the level of cortisol. The best results in the treatment of carbohydrate disorders are obtained by reduction of cortisol level. Unfortunately, not all patients obtaining remission of Cushing’s syndrome gain the state of normoglycaemia. In the case of an acute severe hyperglycaemia, application of metyrapone therapy brings a fast enough effect on reducing cortisol, while in control of high glucose levels multiple insulin injections are recommended. In chronic treatment of carbohydrate disorders the aim is to achieve good metabolic control of diabetes and reduction of HbA1c value. Sometimes, a good effect can be obtained with oral agents, especially those that reduce insulin resistance, such as metformin and PPAR-γ. Furthermore, thiazolidinediones have been demonstrated to have an ACTH-lowering effect on corticotropinomas in in vitro studies. GLP-1 receptor analogues and gliptins could be helpful in the management of hyperglycaemia in CS by increasing glucosedependent insulin secretion and reducing glucagon secretion, as well as having positive effects on appetite, or favourable better fat distribution. Incretin-based medications have also been suggested as the best way to manage pasireotide-induced hyperglycaemia, since somatostatin receptor inhibition leads to reduced secretion of GLP-1 and insulin. If oral hypoglycaemic agents are not effective, combination therapy with insulin is introduced [27].

Hypopituitarism

Adult patients with growth hormone deficiency are characterised by elevated serum concentrations of LDL-cholesterol and triglycerides, lowering of HDL-cholesterol, and increased deposit of abdominal fat. Markers of metabolic syndrome found in this group are twice as likely as in the general population. Abnormal glucose metabolism in GH deficiency results from increasing insulin resistance, but also from impairment of islet β-cell function. All of the listed disorders predispose to an increase in markers of inflammation, development of oxidative stress, and increased cardiovascular risk [28]. Adverse metabolic effects are observed also in patients who complete treatment of GH at the end of the growth process. They have an increased accumulation of body fat, higher blood pressure, and atherogenic lipid profile. There are therefore indications that GH treatment should be continued in adult patients even with only partial growth hormone deficiency. This can lead to improved metabolic state and thereby help to reduce the risk of cardio-vascular events in the future [29]. The substitution effect of growth hormone on glycaemia depends on the duration with GH therapy. In the short period of observation (up to one year of treatment) appears to be adversely affected by reducing peripheral glucose uptake and increasing insulin resistance. This is followed by a glucose tolerance recovery. Improvement in insulin sensitivity may also be obtained by the use of low-dose treatment of growth hormone, with normalisation of IGF-1. Increasing the dose of GH can produce adverse metabolic effects similar to those observed in acromegaly [30]. Abs et al. found that GH treatment did not reduce the risk of developing type 2 diabetes in lean patients with hypopituitarism [31]. In contrast, the application of supplementation in severe GH deficiency, with marked of atherosclerosis, improved metabolic parameters and had a positive impact on the reduction of insulin resistance [32].

Herrmann et al. showed that in the case of fasting hyperglycaemia in patients with GH deficiency, treatment with either metformin alone or in combination therapy with growth hormone, brings favourable metabolic effects [33]. Moreover, co-administration of GH with metformin increases the level of adiponectin, which positively correlates with insulin sensitivity [34].

Among obese patients with hypopituitarism, increased levels of leptin and glucagon-like peptide (GLPs) responsible for the neuroendocrine regulation of food intake were demonstrated [35].

Damage to the hypothalamic-pituitary axis can lead to extreme obesity and related metabolic consequences. After surgery of craniopharyngioma obesity develops in 50% of patients, mainly due to an uncontrolled increase in appetite because the hypothalamus is the central control of energy consumption. Nuclei of the hypothalamus are affected by peptides related to the control of metabolism, such as leptin, insulin, peptide Y (PYY), and ghrelin, but also glucose, free fatty acids, and amino acids. Insulin and leptin are the main regulators of food intake. Damage to the central nuclei of the hypothalamus is responsible for insulin resistance and leptin resistance. Leptin regulates the sensation of hunger by acting on the hypothalamus, inducing the expression of genes encoding pro-opiomelanocortin (POMC), inhibiting the expression of NPY and AGRP (agouti-related peptide) responsible for the feeling of satiety. Damage to hypothalamus nuclei may also result in disinhibition of the vagus nerve impulse and thus in increased stimulation of pancreatic β-cells, hyperinsulinaemia, and obesity [36].

Thyroid diseases

Hyperthyroidism

Approximately 50% of patients with overt hyperthyroidism have impaired glucose tolerance, and in 2–3% diabetes is diagnosed. The diabetogenic effect if thyroid hormones is multifactorial. In the states of thyrotoxicosis, we observed an increased peripheral and hepatic insulin resistance. Simultaneously, an increase in endogenous production of insulin with elevated peripheral degradation of this hormone is observed [37]. Furthermore, thyroid hormone excess leads to enhancement of endogenous glucose production by increased gluconeogenesis and glycogenolysis in the liver. This is probably due to increased expression of the glucose transporter GLUT-2 protein in hepatocytes [38, 39]. Thyroid hormones also affect the lipolysis and therefore increase production of free fatty acids. Moreover, they cause increased glucose absorption from the gastrointestinal tract, which may result from the accelerated gastric emptying and increased blood flow in the portal circulation. This phenomenon is responsible for the development of postprandial hyperglycaemia [40]. In the state of thyrotoxicosis an increased metabolism of glucose in muscle tissue is also observed, mainly by intensifying the anaerobic production of lactic acid in the liver, which becomes the product for gluconeogenesis. Hyperthyroidism increases the concentration of cytokines and markers of inflammation such as IL-6 and TNF-alpha, which also correlate with peripheral insulin resistance. It has been shown that even subclinical hyperthyroidism is characterised by an increase in HOMA-IR. The degree of glucose metabolism disorders correlates with the level of thyroid hormones. Patients with pre-existing diabetes in hyperthyroidism need increased doses of hypoglycaemic agents or insulin and intensification of treatment of diabetes [41].

Hypothyroidism

A deficiency of thyroid hormone promotes weight gain, atherogenic lipid profile, and an increase in blood pressure. Moreover, increased concentration of free fatty acids, reduction in tissue uptake of glucose, and its enhanced oxidation are observed. Authors of previous publications confirmed that in overt and even in subclinical hypothyroidism, a reduced sensitivity of tissues to insulin is present. In the state of thyroid hormone deficiency we also observed an increased production of counter-regulating hormones with potentially diabetogenic properties such as cortisol, catecholamines, and glucagon [42–44]. A positive correlation between thyroid hormone levels and insulin sensitivity has been shown. Triiodothyronine action at the tissue leads to activation of AMP-kinase, which is responsible for increased glucose utilisation, reduced lipolysis, and gluconeogenesis. Implementation of thyroxine in hypothyroidism improves insulin sensitivity [45].

On the other hand, in hypothyroidism lower intestinal absorption of glucose, reduced hepatic and tissue glycogenolysis, and decreased insulin secretion is observed. This effect may result in recurrent hypoglycaemia and reduced insulin requirement among people with previously diagnosed diabetes during hypoglycaemic therapy [37].

Goitre and thyroid cancer

Reduced insulin sensitivity is also a risk factor for increased thyroid volume and nodular goitre. Hyperinsulinaemia exhibits a mitogenic effect on proliferation of thyrocytes. People without diabetes with nodular goitre, despite similar age, sex, BMI, waist circumference, TSH levels, and metabolic parameters, were characterised by higher HOMA-IR [46]. Experimental studies have shown that metformin via activation of AMP-kinase and mTOR (mammalian target of rapamycin) exhibits inhibitory effects on the growth of thyroid cells and antagonises the pro-growth effect of insulin [47]. In the population of patients with insulin resistance, metformin therapy resulted in a reduction in volume of thyroid and nodules [48]. In addition, Ittermann et al. showed that type 2 diabetes patients treated with metformin have a lower thyroid volume compared to those treated with other hypoglycaemic agents [49]. Metformin also reduces the level of serum TSH, the main growth factor for thyroid cells [50].

It was found that the larger vascularisation of nodules in the thyroid gland also depends on the degree of insulin resistance [51]. Going further, patients with papillary thyroid cancer presented more features of metabolic syndrome, and HOMA-IR correlated with the size of tumour [52]. In addition, treatment with metformin in patients with type 2 diabetes and papillary thyroid cancer caused better response to treatment and a higher degree of remission of cancer [53]. It has been shown that metformin antagonised the carcinogenic effect of insulin by cell cycle arrest and inhibition of the clonal growth of cancer cells [54].

Parathyroid diseases

Primary hyperparathyroidism

It is interesting that diabetes develops in approximately 8% of patients with primary hyperparathyroidism, which is three times more often than in the general population [55]. Probably the long-term status of hypercalcaemia and hypophosphataemia triggers insulin resistance and hyperinsulinaemia, and reduces the number of insulin receptors [56]. The surgical treatment of parathyroid adenoma in the early stages of the disease improves glucose tolerance a few weeks after surgery [57].

Disorders of the adrenal gland

Pheochromocytoma

About half of patients with pheochromocytoma have impaired carbohydrate metabolism. In most cases, these tumours produce adrenalin, which by its affinity to the β2 receptors, inhibits insulin secretion, stimulates the secretion of glucagon from the pancreatic islet cells, and decreases glucose uptake by skeletal muscles as well as increases gluconeogenesis and glycogenolysis in hepatocytes [58]. Catecholamines may also cause the phenomenon of insulin resistance by stimulating β receptors and the effect of epinephrine. Lower concentrations of adiponectin and higher HOMA-R in this group of patients with pheochromocytoma were shown [59]. It has been observed that only surgical removal of the lesion can reduce the disorders of glucose metabolism. Therapy with adrenergic α- or β-blocking agents provides only a little therapeutic effect [60]. Patients with pheochromocytoma have less content of both abdominal and subcutaneous fat, and higher HDL-cholesterol concentration, because catecholamines increase lipolysis by activating the adrenergic receptors. After the surgery patients gain weight although the hyperglycaemic effect of catecholamines disappears [61].

Primary aldosteronism

Supraphysiological release of aldosterone in adenoma or adrenal hypertrophy is often associated with the development of disorders of glucose metabolism. The effect of hypokalaemia, which impairs insulin secretion, is well known, but suitable potassium supplementation only partially improves glucose tolerance. Aldosterone alone decreases glucose-dependent insulin secretion. Activation of the mineralocorticoid receptor results in decrease of insulin sensitivity in fat and muscle tissue [62]. Patients with primary aldosteronism have a significantly higher HOMA-R and lower serum adiponectin concentration compared to those with hormone-independent hypertension. It was also shown that fatty liver disease is associated with high aldosterone levels [63]. Surgical treatment of adrenal adenoma resulted in normalisation of serum aldosterone and improvement of insulin sensitivity. In cases of hypertrophy of adrenal glands or contraindications for surgical treatment, spironolactone or eplerenone are used. It has been shown that introduction of spironolactone treatment brings improvements in the reduction of blood pressure but has no positive effect on carbohydrate disorders. This explains the mechanism of action of those agents, which blocks the mineralocorticoid receptors, but at the same time raises the level of aldosterone [64].

Subclinical Cushing’s syndrome and adrenal incidentalomas

It has been shown that non-diabetic individuals, but with subclinical Cushing’s syndrome in the course of adrenal adenoma or even with hormonally inactive incidentaloma, have significantly decreased insulin sensitivity compared to a matched clinical control group. Parameters evaluating insulin resistance did not differ between patients with subclinical Cushing’s syndrome and hormonally inactive adenoma. It was observed that slightly elevated levels of cortisol influence the higher rate of glucose intolerance in the group with subclinical Cushing’s syndrome. It is probable that resection of an accidentally detected adrenal tumour may improve insulin sensitivity and thereby reduce mortality from cardiovascular causes [65].

Congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is known to come from a group of diseases with autosomal recessive mutations of enzymes necessary for the synthesis of cortisol. The most common form of CAH is associated with 21-hydroxylase deficiency (90–95% of cases). Clinical symptoms in classical forms of CAH in adults result from adrenocortical and adrenal medulla insufficiency, hyperandrogenism to virilisation in women, and side effects for the treatment of glucocorticoids [66]. The impairment of cortisol synthesis leads to excessive stimulation of ACTH and adrenal hypertrophy. In addition, patients with the classic form of CAH more often present obesity, hyperinsulinaemia, insulin resistance, and hyperleptinaemia than in the general population. They also demonstrate higher systolic and diastolic blood pressure and lack of a physiological drop in systolic blood pressure at night [67]. These abnormalities contribute to the development of metabolic syndrome and its consequences. Impairment of catecholamine secretion by the adrenal medulla and reduced activation of β-adrenergic receptors leads to lack of inhibition of insulin secretion. Decreased activity of the sympathetic nervous system causes impairment of lipolysis, thermogenesis, and consequently leads to the excessive accumulation of body fat [68]. In people with the classic form of CAH achievement of decreased androgen production requires the use of higher doses of steroids, which causes additional adverse metabolic events [69].

Increased insulin levels and insulin resistance in women with CAH are responsible for the development of secondary polycystic ovarian syndrome (PCOS). It has been shown that up to 75% of women with the classic form of CAH meet the criteria of PCOS [70]. They also have a much higher risk of developing gestational diabetes [71].

Patients with CAH require not only constant monitoring of hormonal parameters, but also clinical and metabolic control such as weight, waist circumference, blood pressure, blood glucose, lipids, and densitometric evaluation of bone mineral density. The use of pharmacological and behavioural methods aimed to decrease obesity, insulin resistance, and metabolic syndrome may also reduce the risk of diabetes and death due to cardiovascular events [72]. In women with type 2 diabetes and non-classical form of CAH, metformin treatment resulted not only in the reduction of glucose levels and improvement of metabolic parameters, but also decreased the level of 17-OH progesterone, and total and free testosterone [73]. It was also observed that pioglitazone in patients with CAH improved insulin sensitivity evaluated by the insulin euglycaemic clamp and had a positive effect on the reduction of blood pressure [74].

Diseases of the gonads

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrine disease among women in reproductive age. It manifests mainly menstrual disorders and impaired fertility, clinical and biochemical hyperandrogenism, and characteristic polycystic image of ovaries in ultrasound scan [75]. Insulin resistance and elevated insulin levels may be an important element in the pathogenesis of PCOS. Hyperinsulinaemia increases the activity of the hypothalamus-pituitary-adrenal axis, resulting in an increase of the androgens secretion. By increasing the number of receptors for luteinising hormone (LH) and IGF-1 in ovaries, enhanced production of testosterone and increased recall cell proliferation occurs. Elevated levels of hormones such as testosterone, insulin, and IGF-1 blocks the maturation of follicles in the dominant follicle and inhibits the occurrence of ovulation. Additionally, hyperinsulinaemia inhibits SHBG synthesis, which in turn leads to increased levels of biologically active fraction of free testosterone [76]. In women with PCOS insulin resistance is rather of a post-receptor nature. However, the pathogenesis of PCOS takes into account the genes involved in the action of insulin, such as insulin receptor gene, the VNTR gene (scattered fragments of the insulin gene), the IGF-1 gene, and IGF-binding protein gene 1 (IGF-1BP) [77]. Due to the increased risk of developing diabetes type 2, cardiovascular diseases, and cancer in patients with PCOS, it seems it should be considered more broadly than just as an endocrine-gynaecological disorder, through the prism of metabolic consequences. The prevalence of metabolic syndrome in women with PCOS depends not only on genetic but also on behavioural factors. Weight loss by lowering insulin levels causes a decrease in androgen production and may lead to the return of ovulation [75]. On the other hand, the therapy with metformin appears to be effective in inducing ovulation and returning impaired fertility in PCOS. Women with PCOS often become pregnant when using dual therapy with metformin and clomiphene. Metformin is also used in the treatment of insulin resistance associated with hyperandrogenism, which manifests as hirsutism and acne. Further observation of the influence of metformin on prevention on type 2 diabetes, cardiovascular disease, or cancer of the endometrium in PCOS is needed [76]. Although the use of GLP-1 agonist liraglutide in the treatment of PCOS women seems promising, the clinical benefits of such therapy require further study. The results indicate that several months’ treatment of liraglutide leads to a decrease in BMI and testosterone levels [78].

Hypogonadism

Decreased level of sex hormones in the course of various forms of hypogonadism is also associated with a risk of developing metabolic syndrome and obesity. Many type 2 diabetic men have lower testosterone levels and symptoms of hypogonadism [79, 80]. Insulin resistance was found in 92% of men with hypogonadism. In such cases, replacement therapy with testosterone brought a rapid improvement in insulin sensitivity [81]. Kapoor et al. have found that in patients with type 2 diabetes and concomitant testosterone deficiency androgen therapy results in an improved metabolic control of diabetes and reduced HbA1c value [82].

Primary hypogonadism present in abnormal karyotype 47 XXY in Klinefelter syndrome is also associated with a high risk of developing abdominal obesity, metabolic syndrome, and type 2 diabetes in adulthood. Markers of insulin resistance can already be seen in boys with this syndrome. Young men are usually phenotypically defined as tall and thin but with a gynoid distribution of adipose tissue and excessive accumulation of abdominal fat [83]. Substitutional therapy with testosterone in this group of patients only partially reverses adverse metabolic effects, but still an increased mortality from cardiovascular disease remains. Therefore, treatment aimed at reducing insulin resistance should be introduced as early as possible.

Corrgian et al. have also shown that women with premature ovarian failure (POF), although they are slimmer compared to properly menstruating women, present lower insulin sensitivity [84].

Women with primary hypogonadism in Turner syndrome often develop obesity, diabetes, and premature atherosclerosis. They present more additional markers of the metabolic syndrome such as hypertriglyceridaemia and higher levels of inflammatory markers (CRP and interleukin-6). These women are characterised by lower levels of insulin and leptin and lower fasting glycaemia in comparison with the eugonadal population. The observed impaired glucose tolerance results from disturbed secretion of insulin [85].

Conclusions

Endocrine disorders induce changes in tissue sensitivity to insulin. The phenomenon of insulin resistance associated with a relative deficiency of insulin secretion leads to hyperglycaemia. Glucose metabolism disorders in the course of endocrinopathy are defined in the aetiological classification of diabetes. Experimental and clinical studies have increased our understanding of the pathomechanism of these relations and should be reflected in the diagnostic and therapeutic procedures.