Alteration of the circadian rhythm of intact parathyroid hormone following a 96‐hour fast

Summary OBJECTIVE PTH(1‐84) secretion in normal male subjects follows a circadian rhythm. The control of this rhythm is multifactorial with both neuroendocrine and chemical influences. The aim of this study was to assess the effect of a 96‐hour fast on the circadian rhythm of PTH(1 ‐84), serum calcium, phosphate and nephrogenous cAMP (NcAMP), an index of PTH(1‐84) bioactivity. DESIGN Blood samples for estimation of all analytes were obtained over a 24‐hour period at 30‐minute intervals. Urine samples were obtained 4 hourly during the daytime and overnight. Each subject was studied on two occasions after being randomized to either (a) normal hospital diet or (b) a 96‐hour fast with water freely available. SUBJECTS Six healthy adult males aged between 28 and 40 years, mean 32 years. MEASUREMENTS PTH(1‐84) was measured by an in‐house immunoradiometric assay. Serum calcium, phosphate, albumin, creatinine and urinary creatinine were measured by standard automated techniques. Calcium was adjusted for albumin. Plasma cAMP was estimated by a commercial method and urine cAMP by in‐house radioimmunoassay and NcAMP obtained by calculation. Rhythm parameters were analysed by cosinor techniques. RESULTS There were alterations in the circadian rhythms of serum phosphate, PTH(1–84) and NcAMP following a 96‐hour fast. Fasting abolished the nocturnal rise in phosphate, PTH(1‐84) and NcAMP but had little effect on the pattern of adjusted calcium over a 24‐hour period. The mean concentrations of serum phosphate, adjusted calcium and NcAMP decreased significantly following the fast and mean PTH(1‐84) increased during day time. CONCLUSIONS Fasting for 96 hours significantly alters the circadian rhythm of PTH(1‐84) secretion by lowering the mean calcium concentration and attenuating the circadian rhythm of serum phosphate.