Effect of thyroxine replacement therapy on plasma insulin-like growth factor 1 levels and growth hormone responses to growth hormone releasing factor in hypothyroid patients.

Abstract

The aim of this study was to evaluate the effect of T4 replacement therapy on plasma insulin-like growth factor 1 (IGF-1) levels in patients with primary hypothyroidism to see whether recovery of pituitary GH responsiveness to GRF was associated with increased plasma IGF-1 levels. IGF-1 levels and GH responses to GRF (1 microgram/kg) were measured in 21 patients with primary hypothyroidism before and after T4 replacement therapy. T4 increased plasma IGF-1 levels (57.2 +/- 4.4 vs 75.9 +/- 8.8 ng/ml, mean +/- SEM, P less than 0.05) and GH responses to GRF as assessed both by peak GH levels (9 +/- 1.5 ng/ml before treatment vs 16.7 +/- 3 ng/ml after treatment, mean +/- SEM, P less than 0.05) and area under curve (496 +/- 92 before treatment vs 896 +/- 161 after treatment, mean- +/- SEM, P less than 0.05). Linear regression analysis showed a positive correlation between free T3 and IGF-1 levels after treatment (r = 0.37, P less than 0.05) and a negative relationship between plasma IGF-1 levels before treatment and delta IGF following T4 replacement therapy (r = 0.45, P less than 0.025). However, no correlation was found between plasma IGF-1 levels and GH responses to GRF, suggesting that GH responses to GRF are of no predictive value in relation to the recovery of plasma IGF-1 levels following T4 replacement therapy in hypothyroid patients.