Absent growth hormone response to L-tryptophan in acromegaly

In acromegaly, regulation of GH secretion by dopamine pathways appears to be qualitatively abnormal. To determine whether regulation of GH secretion by serotonin pathways is also abnormal in acromegaly, we administered L-tryptophan (5 g orally), the initial precursor of serotonin, to 10 patients with active acromegaly (9 treated and 1 untreated), 3 patients with cured acromegaly, and 8 normal subjects. The normal group showed a significant (P less than 0.05) increase in serum GH after L-tryptophan [peak value, 12.3 +/- 4.0 (se) ng/ml], though the magnitude of the response was highly variable. In contrast, subjects with active acromegaly did not show an increase in serum GH after L-tryptophan [mean integrated percentage change in serum GH, -25 +/- 25% (SE); P = NS]. One patient whose acromegaly had been surgically cured did show a GH rise after L-tryptophan. In acromegaly, the GH response to L-tryptophan is absent, suggesting that regulation of GH secretion by serotonin pathways might be qualitatively abnormal.