The impact of insulin resistance on thyroid function and the prevalence of thyroid follicular nodular disease in pregnant women.

Introduction: Insulin resistance (IR) is a phenomenon commonly observed in pregnancy. Increased insulin concentrations might impact thyroid function and structure during gestation.

Objectives: This study investigates the bidirectional relationship between IR indices and thyroid function and morphology in pregnant women.

Methods: In 1,069 gravid participants of the Polish National Programme for Elimination of Iodine Deficiency (2007-2017), blood samples were analyzed for thyroid-stimulating hormone (TSH), FT3, FT4, aTPO, fasting glucose and insulin concentrations, and the thyroid structure was assessed with ultrasound (in 1,065 subjects). Based on calculated homeostatic model assessment of insulin resistance (HOMA-IR) values, participants were stratified into two subgroups: HOMA-nl (HOMA-IR <2.5) and HOMA-h (HOMA-IR ≥2.5), comprising 894 and 175 women, respectively.

Results: Significant difference in mean TSH (1.77 ± 1.17 vs 1.96 ± 1.04; P = 0.008) and mean FT4 (12.65 ± 2.3 vs 11.47 ± 1.9; P = 0.001) concentrations between HOMA-nl and HOMA-h groups was found. The subgroups did not differ in thyroid nodularity or multinodular goiter prevalence. HOMA-IR positively correlated with TSH concentrations, BMI and thyroid volume. Serum FT3 and FT4 concentrations showed negative correlations with HOMA-IR.

Conclusions: IR seems to affect the thyroid function of gravid women by diminishing the ability to respond to increased thyroid hormone demand. Thyroid volume increase during pregnancy may be influenced by IR; however, its short-term effect on thyroid nodularity appears to be negligible.