INFRADIAN RHYTHMS IN URINARY GROWTH-HORMONE EXCRETION

All studies of urinary GH excretion in normal and disordered growth ha c-e revealed marked day to day (infradian) variation. We used serial o vernight urinary GH estimations as an indirect measure of endogenous G H secretion in eight normal prepubertal children (aged 3.6-7.3 yr) ove r 90-365 days to determine whether longer term rhythms in GH output co uld exist. This study constitutes a first step in examining the potent ial relationship between GH excretion and growth. Urinary GH was measu red by immunoradiometric assay after dialysis, ex-pressed as the total amount excreted (nanograms per night) or as the GH/creatinine ratio ( nanograms per mmol), and assessed by pulse counting techniques and tim e-series analysis. Variability in urinary GH excretion (median coeffic ient of variation, 46%) was significantly greater than creatinine (med ian coefficient of variation, 25%; P = 0.003). Additionally, there was marked month by month variation in baseline urinary GH in all childre n. High frequency pulses of urinary GH were defined in all children, w ith periods between 3-5 days. In the two children followed for 7 month s or more, time-series analysis was also undertaken on urinary GH data divided into weekly series. This revealed significant rhythms present at 2.6 and 4.1 weeks. There were, therefore, three components to urin ary GH excretion: long term basal fluctuation (over months), short ter m pulses (over days), and intermediate rhythms (over weeks). Further w ork is required to establish the relationship between these patterns o f GH excretion and short term growth.