Silent somatotroph adenomas of the human pituitary. A morphologic study of three cases including immunocytochemistry, electron microscopy, in vitro examination, and in situ hybridization.

Pituitary adenomas, removed surgically from three women with normal or slightly elevated serum growth hormone levels and no evidence of acromegaly, were studied. The tumor cells were shown by electron microscopy to correspond to sparsely granulated somatotrophs but immunocytochemistry showed that they contained no, moderate, or little growth hormone. Two tumors examined in vitro secreted small amounts of growth hormone in the tissue culture medium initially with a spontaneous rise after several days, and responded to growth hormone-releasing hormone stimulation with increased growth hormone release. In situ hybridization demonstrated growth hormone mRNA expression in adenoma cells. Clinically silent somatotroph adenomas represent a hitherto undescribed entity; electron microscopy shows that they consist of somatotrophs, and express growth hormone mRNA but do not secrete growth hormone in amounts needed to raise substantially serum growth hormone levels and cause acromegaly. Further work is required to clarify the mechanisms accounting for the lack of clinical and biochemical evidence of hormone excess associated with these tumors.