Introducing the thyroid gland as another victim of the insulin resistance syndrome

Insulin is a thyroid growth factor that stimulates proliferation of thyroid cells in culture. In order to evaluate the effects of insulin resistance (IR) on the thyroid gland, we developed a prospective study in euthyroid women.

One hundred eleven women (mean age 32.2 +/- 7 years) were evaluated by a thyroid ultrasound (US) and basal and postprandial serum insulin. Subjects were divided into four groups as follows: G1 (n = 42), subjects with IR and obesity; G2 (n = 21), subjects with obesity without IR; G3 (n = 17), subjects with IR and normal weight; and G4 (n = 31) control group (without IR and obesity).

The thyroid volume (TV), measured by US, showed the following values: G1, 17 +/- 3 mL; G2, 13.8 +/- 2.8 mL; G3, 16.2 +/- 2.1 mL; and G4,12.1 +/- 2.4 mL. There was no significant difference in TV between G1 and G3, but differences between G1 and G2, and between G3 and G4 were significant at p < 0.05. The percentage of nodular thyroid glands observed by US in each group was as follows: G1, 50%; G2, 23.8%; G3, 61%; G4, 16.1%. Again, the differences between G1 and G2 and between G3 and G4 were statistically significant (p < 0.005 and p < 0.001, respectively, for each comparison).

It is concluded that the higher circulating levels of insulin cause increased thyroid proliferation. The clinical manifestations are the larger thyroid volume and the formation of nodules. Thus, the thyroid gland appears to be another victim of the insulin resistance syndrome.