Evaluation of glucose-suppressed GH and IGF-I levels in patients with active and inactive acromegaly

Objectives: In order to biochemically assess disease activity in patients with acromegaly, nadir growth hormone (GH) levels after oral glucose tolerance testing (OGTT) as well as random insulin-like growth-factor I (IGF-I) levels were used. Until recently nadir GH levels below 1 or 2µg/L were used as a diagnostic criterion for adequate control. As new, more sensitive GH assays are developed, a critical re-evaluation of these traditional cut off values is necessary. Recently, some investigations suggested that nadir GH levels after OGTT with values of 0.33µg/L indicate inactive acromegaly.

Methods: In a cross sectional study, we retrospectively evaluated nadir GH concentrations during OGTT and corresponding IGF-I levels before glucose load in a series of 67 acromegalic patients (31f/36m, median age 53y (range 20–75)). 58 of the 67 patients underwent surgery. At the time point of biochemical evaluation, 23 patients were on medication with a somatostatin analogue and 2 patients were received a dopamine agonist. GH levels were measured 0, 30, 60, 120 and 180min following glucose (75g) administeration. Concentrations of GH and IGF-I were determined by a single lab using the same chemiluminescence immunoassays (Nichols Advantage, San Clemente, CA). For analysis, IGF-I levels are given as multiples of the upper limit of normal [xULN].

Results:

	xULN ≤ 1			xULN >1
GH	≤ 0.3	0.4–1.0	>1.0	≤ 0.3	0.4–1.0	>1.0
n	25	10	9	0	8	15
Med	3	7	3	0	3	8
Op	25	10	9	0	5	9

GH=GH nadir after OGTT (µg/L), Med: pat on medication including pat with and without surgery, Op=pat after surgery including pat with and without medication

Conclusion: In our series, all patients with a GH nadir ≤0.3µg/L showed biochemical remission as judged by xULN ≤ 1. For GH nadir >0.3µg/L it is not possible to clearly differentiate between residual activity and remission. However neither patients' history nor current medication or status of glucose metabolism provided an explanation for the discordant results in patients showing normal xULN, but elevated GH nadir after OGTT.