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

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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 and GH responses to GRF (1 .mu.g/kg) were measured in 21 patients with primary hypothyroidism before and after T4 replacement therapy. 4 increased plasma IGF-1 levels (57.2 .+-. 4.4 vs 75.9 .+-. 8.8 ng/ml, mean .+-. SEM, P < 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 < 0.05) and area under curve (496 .+-. 92 before treatment vs 896 .+-. 161 after treatment, mean .+-. SEM, P < 0.05). Linear regression analysis showed a positive correlation between free T3 and IGF-1 levels after treatment (r = 0.37, P < 0.05) and a negative relationship betwen plasma IGF-1 levels before treatment and .DELTA.IGF following T4 replacement therapy (r = 0.45, P < 0.025). However, no correlation was found between plasma IGF-1levels and GH responses to GRF, suggesting that GH responses to GRF are of no predictive value in relation to the recovery of plasm IGF-1 levels following T4 replacement therapy in hypothyroid patients.