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.

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.