Association between serum insulin-like growth factor-I levels and thyroid disorders in a population-based study

Objective: There is current debate on whether serum IGF-I levels are associated with thyroid disorders. The aims of the present study were: 1) to investigate possible associations between serum IGF-I levels and thyroid disorders and 2) to analyze the role of serum IGF binding protein (IGFBP)-3 and TSH levels for these associations. Design: This was a cross-sectional Study of Health in Pomerania. Setting: The study was conducted in the general population of northeast Germany. Subjects: The study population comprised 3662 subjects (1746 women) without history of thyroid disorders. Interventions: No interventions have been performed. Main Outcome Measures: Goiter and thyroid nodules were determined by ultrasound. Serum TSH levels less than 0.25 mIU/liter were considered decreased. Results: Adjusted for major confounders and risk factors for thyroid disorders, subjects with serum IGF-I levels above the upper tertile had higher odds for goiter relative to subjects with serum IGF-I levels below the lower tertile [odds ratio (OR) 1.67; 95% confidence interval (CI) 1.24-2.26 in women; OR 2.04; 95% CI 1.55-2.68 in men]. A similar association was present for thyroid nodules in men (OR 1.64; 95% CI 1.17-2.32) and for decreased serum TSH levels in women (OR 1.65; 95% CI 1.00-2.69). Serum IGFBP-3 levels were not associated with thyroid disorders and did not represent effect modifiers for the association between serum IGF-I levels and the endpoints. Conclusions: We conclude that high serum IGF-I levels are associated with goiter. Whereas high serum IGF-I levels are also related to thyroid nodules in men, they are related to decreased serum TSH levels in women. Serum IGFBP-3 and TSH levels did not modulate these associations.