EFFECT OF PYRIDOSTIGMINE AND PIRENZEPINE ON GH RESPONSES TO GHRH IN HYPERTHYROID PATIENTS

OBJECTIVE We wished to investigate whether thyrotoxicosis can influence the cholinergic modulation of GH secretion. DESIGN Pyridostigmine was given orally, then GHRH injected i.v., and levels were measured. In a separate study, pirenzepine was injected i.v., then GHRH, and growth hormone levels were measured. PATIENTS Thyrotoxic patients were compared with normal subjects. MEASUREMENTS GH was measured from -30 to +120 minutes at intervals of 15 minutes. RESULTS Pyridostigmine markedly increased GH responses to GHRH in normal subjects, but not in thyrotoxic patients. Pirenzepine abolished the GH response to GHRH in thyrotoxic patients. CONCLUSIONS GH responses to GHRH in hyperthyroid patients were suppressed by cholinergic muscarinic receptor blockade with pirenzepine. Activation of cholinergic pathways with pyridostigmine did not increase GH responses to GHRH in these patients. This may be a consequence of increased hypothalamic cholinergic function or reduced hypothalamic GHRH activity in hyperthyroidism. Our findings demonstrate a further mechanism by means of which thyroid status may affect the secretory activity of the somatotroph.