Parathyroid.

Parathyroid hormone is the main stimulus for the kidney enzyme that converts 25-hydroxyvitamin D into the active 1,25-dihydroxy form. The active hormone then raises calcium absorption from the gut and feeds back to suppress PTH release. The two sit in a closed loop, which is why PTH is measured whenever vitamin D status is being assessed.

The calcium-sensing receptor on the parathyroid cell reads ionised calcium in the blood minute to minute. A fall releases PTH within seconds, a rise shuts it off. Calcium intake therefore sets the tone of the whole axis, and a low-calcium diet produces a chronically raised PTH even when vitamin D is adequate.

Severe magnesium depletion blocks PTH release from the gland and blunts the response of bone and kidney to circulating hormone. The result is a low calcium that does not correct until magnesium is replaced, which catches people out because calcium supplementation alone fails. Mild magnesium shortfall does not do this.

Rising serum phosphate stimulates PTH release both directly and by lowering ionised calcium and suppressing active vitamin D. PTH in turn increases renal phosphate excretion, which is the corrective arm of the loop. High phosphate intake, common with processed food and cola, pushes the axis in the direction of higher PTH.

Active vitamin D drives osteoblasts to make osteocalcin, but the protein only binds calcium into the bone matrix once vitamin K dependent carboxylase has modified its glutamate residues. Without adequate vitamin K a portion of circulating osteocalcin stays undercarboxylated. The link to bone density outcomes in people is less settled than the biochemistry.

Boron supplementation has been reported to slow the breakdown of vitamin D metabolites and shift calcium and magnesium handling in small studies. The mechanism is not well characterised and the human work is limited in size. Read it as a signal worth noting rather than an established relationship.

Potassium citrate supplementation shifted bone turnover markers in postmenopausal women, with the proposed mechanism being reduced acid load and lower urinary calcium loss. Bone turnover markers are markers, not fracture outcomes. The direction is favourable but the endpoint is a surrogate.

Routine versus selective calcium supplementation after total thyroidectomy was compared in a randomised trial, because handling of the gland during surgery can transiently disturb calcium regulation. This is a clinical management question handled by a surgical team, not a self-directed supplement decision.

A systematic review of vitamin D2 found consistent effects on parathyroid hormone, calcium and phosphorus in humans. Both D2 and D3 raise 25-hydroxyvitamin D and suppress PTH, though they differ in potency and half-life. The suppression of PTH is the expected physiological response, not a separate benefit.