Vitamin D metabolism and parathyroid function in man

1. The metabolism of an intravenous pulse dose of double-isotope-labelled cholecalciferol has been studied in control subjects with widely differing states of vitamin D nutrition and in patients with primary disorders of parathyroid function. 2. The formation of labelled 1,25-dihydroxy-cholecalciferol [1,25-(OH)2D3] and labelled 24,25-dihydroxycholecalciferol [24,25-(OH)2D3] has been related to the prevailing concentrations in serum of 25-hydroxycholecalciferol [25-(OH)D3], immunoreactive parathyroid hormonel, calcium and orthophosphate (Pi). 3. In control subjects with relative vitamin D deficiency [serum 25-(OH)2D3 was related inversely to the serum 25-(OH)D3 and serum calcium, and directly to serum immunoreactive parathyroid hormone. No formation of 1,25-(OH)2D3 was detectable to form labelled 24,25(OH)2D3 preferentially. 4. No control subject produced significant amounts of both labelled 1,25-(OH)2D3 and labelled 24,25-(OH)2D3 simultaneously. 5. All subjects with primary hyperparathyroidism produced significant amounts of labelled 1,25-(OH)2D3 and labelled 24,25-(OH)2D3 simultaneously; the renal turnover of 25-(OH)D3 was apparently greater than in nutritionally matched controls. Serum labelled 1,25-(OH)2D3 in this disease was not correlated with serum 25-(OH)D3, immunoreactive parathyroid hormone, calcium or Pi. Production of labelled 24,25-(OH)2D3 was inappropriately high for the prevailing nutritional state. 6. The indirectly estimated their concentration of 1,25-(OH)2D3 showed only a fourfold variation in control subjects (45-180 pmol/l), compatible with its having a regulated hormonal function. 7. The data suggest that the production of 1,25-(OH)2D3 from a pulse dose of cholecalciferol is normally regulated, directly or indirectly, by the parathyroid hormone.