The advantage of measuring spontaneous growth hormone (GH) secretion compared with the insulin tolerance test in the diagnosis of GH deficiency in young adults

Objective Reassessment of GH status after the attainment of adult height has important clinical implications in the diagnosis and prognosis of GH deficiency (GHD) in adulthood. The current GH threshold for biochemical definition of GHD in young adults is still a subject of debate. Design To investigate the role of pharmacological stimulation tests compared with spontaneous 12-h nocturnal GH secretion in the diagnosis of permanent GHD in young adults with childhood-onset GHD. Patients Forty-five young adults (25 males, 20 females) with childhood-onset GHD, height standard deviation score (SDS) -1.1 +/- 1.3 and body mass index (BMI) SDS 1.0 +/- 1.6, were re-evaluated at the age of 19.8 +/- 2.7 years. Sixteen subjects showed a normal pituitary gland on magnetic resonance imaging (MRI), while in 29, consistent structural hypothalamic-pituitary abnormalities were found. GH secretion was assessed by means of an insulin tolerance test (ITT) and a 12-h spontaneous nocturnal profile as well as by IGF-I assessment. The results were compared with those of 43 healthy controls. Results Mean 12-h spontaneous nocturnal GH secretion was < 3.1 mu g/l (the lowest limit of the normal range) in 36 (80%) of the subjects and > 3.1 mu g/l in nine (20%). Of these 36 patients, 29 (80%) had abnormal MRI findings and 20 (55%) had multiple pituitary hormone deficiencies (MPHD). All nine subjects with mean spontaneous GH secretion > 3.1 mu g/l had a normal pituitary MRI, isolated GHD and a peak GH response to ITT > 5 mu g/l. There was a discordance in 14 patients (31%), who showed a peak GH response to ITT > 5 mu g/l but a reduced spontaneous GH secretory capacity; 10 had structural hypothalamic-pituitary abnormalities on MRI. Conclusions Although the ITT provides valuable information and proves to be a sensitive index of permanent GHD, the results of this study emphasize the potential diagnostic value of assessment of 12-h spontaneous GH secretion in young adults with childhood-onset GHD.