PITUITARY ENLARGEMENT DUE TO PRIMARY HYPOTHYROIDISM GROWTH-HORMONE RESPONSE TO GHRH, GHRP-6 AND GHRH PLUS GHRP-6

GH secretion after growth hormone-releasing hormone (GHRH), growth hor mone releasing peptide-6 (GHRP-6) and after combined administration of both peptides was studied in a patient with lactotrope and thyrotrope hyperplasia due to primary hypothyroidism. Pituitary pseudotumor disa ppeared after thyroid hormone replacement; this was evidenced by magne tic resonance imaging (NMR). There was no difference between areas und er the curve (AUC(0-120min)) during GHRH test before and after thyroid hormone replacement (136.5 vs 129.0 mu g/l min). Maximal GH increases over basal values (Delta GH) did not change (1.5 and 1.9 mu g/l). GH secretion induced by GHRP-6 increased after treatment (AUC(0-120min) 1 97.2 vs 650.4 mu g/l min), Delta GH increments were 4.0 and 18.3 mu g/ l before and after therapy respectively. When the peptides were admini stered together a synergistic effect on GH secretion was observed but GH release was much more powerful after pituitary pseudotumor disappea rance (AUC(0-120min) 1043.2 vs 2046.7 mu g/l min). This was accompanie d by increased Delta GH (22.7 vs 35.5 mu g/l) The synergic. action of peptides normalized in euthyroid condition and after the resolution of pituitary pseudotumor mainly due to improved GH response to GHRP-6. B lunted response of GH to GHRP-6 and GHRP-6 plus GHRH were in part due to known effects of hypothyroidism on GH secretion, Hypothalamopituita ry disconnection and/or decrease in the synthesis of an unknown factor in the hypothalamus which mediates the effects of GHRP-6 may have par ticipated in the GH responsiveness of this patient. This case adds to in vivo evidence that GHRP-6 operates through a non-GHRH dependent mec hanism.