Acromegaly with Normal Fasting Growth Hormone Concentrations but Abnormal Growth Hormone Regulation

During the past 6 years we evaluated 62 patients with acromegaly, 5 of whom (8%) had consistently lower growth hormone concentrations than expected in classical acromegaly. Growth hormone dynamics were evaluated by a standard oral glucose tolerance test, intravenous insulin, oral L-dopa (1.0 g), 24-hour random sampling, and nocturnal secretion after the onset of behavioral sleep. Growth hormone concentration did not correlate with clinical manifestations and was less than 10 ng/ml during multiple sampling over a 24-hour interval, except for an occasional growth hormone spike. No nocturnal growth hormone peak or decreased concentration was observed after sleep. Growth hormone secretion was not suppressed during the glucose tolerance test, nor stimulated by L-dopa or intravenous insulin. These responses appear analogous to the hormone dynamics seen in other hyperfunctioning endocrinopathies and suggest autonomous function at the pituitary or hypothalamic level or both. Our evaluation of growth hormone dynamics by multiple random samples, nocturnal secretion, and the response to the glucose tolerance test were particularly helpful in confirming the diagnosis of acromegaly, especially in these five patients.