Thyroxine treatment increases circulating levels of insulin-like growth factor binding protein-1: a placebo-controlled study

OBJECTIVE Thyroid hormones affect carbohydrate metabolism in the liver, and hepatic insulin-like growth factor binding protein-1 (IGFBP-1) participates in glucose counter-regulation, so we studied the effects of oral thyroxine on serum IGFBP-1. DESIGN The study was performed on a placebo-controlled cross-over basis covering 3 months’ thyroxine and 3 months’ placebo administration. PATIENTS Eight patients taking anticonvulsant medication and four patients with hypothalamic hypothyroidism were given thyroxine, 150–200 μg daily for 3 months, or placebo. MEASUREMENTS Serum IGFBP-1, sex hormone binding globulin, free T3 and free T4, TSH and IGF-I levels were measured after an overnight fast before treatment, and at the end of each 3-month period. RESULTS Anticonvulsant medication had no significant effect on serum IGFBP-1. After 3 months’ thyroxine treatment the serum IGFBP-1 levels (69; 58-167 μ g/l; median and interquartile range, respectively) were significantly higher than those after placebo treatment (44; 23-58 μ g/l; P= 0 002), or the pretreatment levels (54; 19-81 μ g/l, P= 0 005). The IGFBP-1 levels rose in all 12 patients after thyroxine treatment, the median rise being 2.1-fold compared to placebo levels. No change was found in serum IGF-I concentrations. CONCLUSIONS Oral thyroxine produces a rise in serum IGFBP-1 level without a change in serum IGF-I concentration.