Introduction: The high prevalence of obesity and thyroid diseases worldwide justifies di per se their simultaneous coexistence. In recent decades, there has been a parallel and significant rise in obesity and thyroid diseases in industrialised countries, although the underlying mechanisms are complex and not well known.

Material and methods: The authors accomplished a comprehensive literature search of original articles concerning obesity and thyroid status. Original papers exploring the association between these two morbidities in children and adults were included.

Results: A total of 79 articles were included in the present analysis. A total of 12% of obese children (mean age 10.9 ± 1.4 years) showed a thyroid disease, and they were younger than healthy obese children (10.9 ± 1.2 vs. 11.0 ± 0.4 years, p < 0.001). Isolated hyperthyrotropinaemia was the most frequent finding in children (10.1%). Autoimmune thyroid disease was more frequent in puberal age. Thyroid antibodies and subclinical hypothyroidism were more frequent in obese that in non-obese patients (7% vs. 3%, p < 0.001; 10% vs. 6%, p < 0.001). Among obese adults, 62.2% displayed a thyroid disease; those affected were younger (35.3 ± 6.8 vs. 41.0 ± 1.9 years, p < 0.001), heavier [body mass index (BMI): 39.4 ± 6.3 vs. 36.1 ± 2.3 kg/m², p < 0.001], and more frequently female (13% vs. 8%, p < 0.001). The most frequent disease was overt hypothyroidism (29.9%). BMI appears to be correlated with TSH levels in obese adults. Overt hypothyroidism was significantly more frequent in obese patients (7% vs. 3%, p < 0.005), but no difference was found in thyroid antibodies (15% vs. 14%, p = 0.178).

Conclusions: An undeniable relationship between obesity and thyroid impairments exists. Isolated hyperthyrotropinaemia is frequently seen in obese children, often followed by spontaneous resolution. Subclinical hypothyroidism should never be treated in children or adults with the aim of reducing body weight.