Calcium, Phosphate & Renal Endocrine

PTH: defending serum calcium

Trigger. Parathyroid chief cells release PTH when serum ionized Ca²⁺ falls, sensed by the calcium-sensing receptor (CaSR). The net effect is to raise serum Ca²⁺ and LOWER serum phosphate, acting directly on bone and kidney and indirectly on the gut through vitamin D.

Magnesium matters. PTH secretion requires Mg²⁺: severe hypomagnesemia paradoxically suppresses PTH release and blunts its action, producing hypocalcemia that will not correct until Mg²⁺ is replaced.

Renal actions. PTH acts on three nephron targets:

• Increases Ca²⁺ reabsorption in the distal convoluted tubule (TRPV5 channels).
• Inhibits proximal tubule phosphate reabsorption by internalizing the Na⁺-phosphate (NaPi) cotransporter, causing phosphaturia; urinary cAMP rises as a marker of its action.
• Stimulates proximal tubule 1α-hydroxylase, raising calcitriol.

Bone actions. PTH acts on osteoblasts, which raise RANKL and lower osteoprotegerin; RANKL activates osteoclasts to resorb bone, releasing Ca²⁺ and phosphate. Because the kidney dumps the phosphate, the net serum result is high Ca²⁺ and low phosphate: PTH is the phosphate-wasting hormone.

Vitamin D and FGF23

Activation. Vitamin D is made in skin or eaten (D2 from plants, D3 from animal foods), then hydroxylated twice:

Actions. Calcitriol raises intestinal absorption of BOTH Ca²⁺ and phosphate, supports bone mineralization, and feeds back to suppress PTH synthesis.

Regulating 1α-hydroxylase. The proximal tubule enzyme is turned ON by PTH and low phosphate and OFF by FGF23, high phosphate, and calcitriol itself (negative feedback). Its loss in CKD is why calcitriol falls. Granulomas (e.g., sarcoidosis) contain macrophage 1α-hydroxylase that escapes this control, causing PTH-independent hypercalcemia.

FGF23, the phosphatonin. Osteocytes and osteoblasts secrete FGF23 when phosphate and calcitriol rise. Acting with its co-receptor Klotho, it causes phosphaturia (down-regulates proximal NaPi) AND inhibits 1α-hydroxylase, lowering calcitriol: it makes you excrete phosphate and lose active vitamin D. Excess FGF23 causes X-linked hypophosphatemic rickets (PHEX mutation) and tumor-induced osteomalacia, with low phosphate, phosphaturia, and inappropriately low calcitriol.

Renal handling of calcium and phosphate

Calcium. About 40% of plasma Ca²⁺ is albumin-bound and not filtered; the ionized (active) and anion-complexed fractions are filtered. Alkalosis increases albumin binding and lowers ionized Ca²⁺. Most filtered Ca²⁺ is reabsorbed early, in the proximal tubule and thick ascending limb, by the paracellular route, driven in the TAL by the lumen-positive potential. Loop diuretics abolish that potential and INCREASE Ca²⁺ excretion. The DCT fine-tunes the rest, and both PTH and thiazides INCREASE Ca²⁺ reabsorption there.

Phosphate. Most filtered phosphate is reclaimed in the proximal tubule by the NaPi cotransporter. PTH and FGF23 both inhibit NaPi, the shared target that explains why either hormone lowers serum phosphate.

Calcitonin. Parafollicular C cells of the thyroid release calcitonin when Ca²⁺ is HIGH; it inhibits osteoclasts and lowers serum Ca²⁺, opposing PTH and toning down bone turnover. Its physiologic role in humans is minor (thyroidectomy does not derange Ca²⁺), but it is the tumor marker for medullary thyroid carcinoma.

The kidney as an endocrine organ

Erythropoietin. Peritubular interstitial fibroblasts of the renal cortex and outer medulla make EPO when they sense hypoxia, which stabilizes hypoxia-inducible factor (HIF) and switches on the EPO gene. CKD causes a normocytic anemia from EPO loss (treated with erythropoiesis-stimulating agents), whereas renal cell carcinoma can secrete ectopic EPO and cause paraneoplastic polycythemia.

Renin. Juxtaglomerular cells, modified smooth muscle cells of the afferent arteriole, release renin, the rate-limiting step of the RAAS, in response to low renal perfusion pressure (they act as baroreceptors), low NaCl at the macula densa, and β1-sympathetic stimulation. Renin cleaves angiotensinogen to angiotensin I. Together with calcitriol and EPO, renin completes the kidney's three classic hormone outputs.

CKD mineral and bone disease

The cascade. Failing kidneys cannot excrete phosphate or make calcitriol. Hyperphosphatemia and low calcitriol both lower serum Ca²⁺ (phosphate also binds free Ca²⁺), and low Ca²⁺, high phosphate, and low calcitriol together drive secondary hyperparathyroidism. FGF23 rises early in the attempt to shed phosphate.

Consequences. Chronic stimulation causes renal osteodystrophy (bone pain, fractures, high-turnover bone disease) and can make the parathyroids autonomous: tertiary hyperparathyroidism, with high Ca²⁺ and very high PTH. A high Ca²⁺ × phosphate product promotes vascular and soft-tissue calcification, and pruritus is common.

Treatment. Dietary phosphate restriction, phosphate binders (sevelamer), calcitriol, and calcimimetics (cinacalcet, which sensitizes the CaSR to suppress PTH).