THE PARATHYROID-HORMONE CIRCADIAN-RHYTHM IS TRULY ENDOGENOUS - A GENERAL CLINICAL RESEARCH-CENTER STUDY

While circulating levels of PTH follow a diurnal pattern, it has been unclear whether these changes are truly endogenous or are dictated by external factors that themselves follow a diurnal pattern, such as sle ep-wake cycles, light-dark cycles, meals, or posture. We evaluated the diurnal rhythm of PTH in 11 normal healthy male volunteers in our Int ensive Physiologic Monitoring Unit. The first 36 h spent under baselin e conditions were followed by 28-40 h of constant routine conditions ( CR; enforced wakefulness in the strict semirecumbent position, with th e consumption of hourly snacks). During baseline conditions, PTH level s followed a bimodal diurnal rhythm with an average amplitude of 4.2 p g/mL. A primary peak (t(1max)) occurred at 0314 h, and the secondary p eak (t(2max)) occurred at 1726 h, whereas the primary and secondary na dirs (t(1min) and t(2min)) took place, on the average, at 1041 and 210 3 h, respectively. This rhythm was preserved under CR conditions, albe it with different characteristics, thus confirming its endogenous natu re. The serum ionized calcium (Ca-i) demonstrated a rhythm in 3 of the 5 subjects studied that varied widely between individuals and did not have any apparent relation to PTH. Urinary calcium/creatinine (UCa/Cr ), phosphate/Cr (UPO4/Cr), and sodium/Cr (UNa/Cr) ratios all followed a diurnal rhythm during the baseline day. These rhythms persisted duri ng the CR, although with different characteristics for the first two p arameters, whereas that of UNa/Cr was unchanged. In general, the tempo ral pattern for the UCa/Cr curve was a mirror image of the PTH curve, whereas the UPO4/Cr pattern moved in parallel with the PTH curve. In c onclusion, PTH levels exhibit a diurnal rhythm that persists during a CR, thereby confirming that a large component of this rhythm is an end ogenous circadian rhythm. The clinical relevance of this rhythm is ref lected in the associated rhythms of biological markers of PTH effect a t the kidney, namely UCa/Cr and UPO4/Cr.