Serum 1,25-(OH)_2-D concentrations in normal volunteers were measured by a sensitive and reliable assay. Serum 1,25-(OH)_2-D level was the highest at mid-teens over 2 y.o. and maintained fairly constant over 18 y.o., without significant declinement in the old people. Neither minutes-to-minutes variation nor clear diurnal variation of serum 1,25-(OH)_2-D were demonstrated. There is significant variation of serum 1,25-(OH)_2-D, when day-to-day values are evaluated and any demonstrable rules for such day-to-day variations of serum 1,25-(OH)_2-D are not present. In addition, individual variation in the degree of changes in serum 1,25-(OH)_2-D is noticed. In conclusion, 1,25-(OH)_2-D is a loosely controlled steroid hormone and a careful interpretation for assayed values and a frequent determination in the serum for 1,25-(OH)_2-D are required.
