Positive association between circulating free thyroxine and insulin‐like growth factor I concentrations in euthyroid elderly individuals

OBJECTIVE: Previous studies have shown marked changes in circulating insulin-like growth factor-l (IGF-l) levels in hypo- and hyperthyroid patients. In this study we examined whether the circulating concentration of IGF-l may also be affected by normal thyroid hormone levels. DESIGN: A cross-sectional study of thyroid hormones and plasma IGF components in a population-based sample. PATIENTS: 50-80 year-old men (n = 262) and women (n = 218) with normal concentrations of serum free thyroxine (fT4), free triiodothyronine (fT3) and thyrotrophin (TSH). MEASUREMENTS: Plasma concentrations of IGF-l, IGF-ll and IGFBP-3, and serum concentrations of TSH, fT3 and fT4. RESULTS: Serum fT4 values were weakly positively correlated and serum TSH levels were inversely correlated with circulating IGF-l concentrations. The associations persisted after adjustment for age and ideal body weight. CONCLUSIONS: Our data demonstrate that thyroid hormones are positively correlated to IGF-l plasma levels even under physiological conditions. However, thyroid hormones explain only 1-2% of the normal variability of circulating IGF-1.