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Positive association between circulating free thyroxine and insulin-like growth factor l concentrations in euthyroid
T Seck1, C Scheidt-Nave, R Ziegler
1Department of Internal Medicine l, University of Heidelberg, Germany.
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.