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Antônio Ribeiro-Oliveira Jr, Alexander T Faje and Ariel L Barkan

Context

Measurement of GH after oral glucose tolerance test (OGTT) is used for the diagnosis and surveillance of acromegaly. However, there are major discrepancies between glucose-suppressed GH and plasma IGF1 as indices of biochemical activity of acromegaly in patients with relatively mild GH oversecretion. This study was aimed to assess the performance of OGTT in patients with acromegaly and variable GH outputs.

Methods

Forty adults with newly diagnosed, untreated acromegaly (15 with GH >4.3 μg/l and 25 with GH <4.3 μg/l) and elevated IGF1 levels were studied. All underwent Q10 min for 24 h sampling for GH followed by an OGTT.

Results

Postglucose nadir GH (GHn) correlated significantly to 24 h GHn, mean 24 h GH, and baseline GH (P<0.001 for all comparisons). GHn correlated significantly to IGF1 z-scores for the ‘low’ GH group and for the entire group (P<0.0001 for both comparisons), but not for the ‘high’ GH group. None of the patients with mean GH >4.3 μg/l had GHn below 1 μg/l. In contrast, 13 out of 25 patients (52%) with GH <4.3 μg/l showed GHn lower than 1 μg/l, and 7 of them (28%) had GHn lower than 0.4 μg/l. These groups did not differ significantly either for average or for maximal GH suppression in OGTT.

Conclusions

Our data show that suppressibility of GH by glucose in acromegaly is a function of the degree of GH hypersecretion and that OGTT has only limited diagnostic value in patients with biochemically active acromegaly but only mildly increased GH output.

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P Jean Ho, Gad B Kletter, Nancy J Hopwood, Roberta DeMott Friberg and Ariel L Barkan

To assess the relative roles of growth hormone-releasing hormone (GHRH) pulse and somatostatin withdrawal as potential generators of pulsatile growth hormone (GH) release in humans, we studied GH responses to iv bolus GHRH (1 μg/kg) and to termination of a 4 h iv somatostatin infusion (7.2 μg·kg−1·h−1) in five normal young men, and in five men with previously diagnosed isolated GH deficiency. The patients were diagnosed 8–15 years previously on the basis of typical auxological and hormonal criteria, were treated with exogenous GH and were off GH therapy for 1.5–8.9 years prior to this study. Growth hormone rises to a bolus GHRH were similar between the controls and the patients (maximum GH 27.3±15.3 vs 8.0±4.0 μg/l). The controls exhibited only a small GH rise to somatostatin withdrawal (maximum GH 2.9±1.2 μg/l), while the patients did not (maximum GH 0.7±0.1 μg/l; p<0.05). We conclude that somatostatin withdrawal by itself is an ineffective promoter of GH pulsatility. Periodic quiescence of somatostatinergic neurons must be associated with a concomitant GHRH pulse in order to result in a robust GH pulse.