Function of PTH on Kidney Tubules

The function of PTH (Parathormone) on kidney tubules is:

Correct Answer: Increase calcium reabsorption & phosphorus excretion

Parathormone (PTH), secreted by the four parathyroid glands, acts on kidney tubules to increase calcium reabsorption and simultaneously increase phosphorus (phosphate) excretion.

Three Sites of PTH Action

• Kidney: ↑ Ca²⁺ reabsorption from DCT; ↑ PO₄³⁻ excretion (phosphaturia); also stimulates 1-hydroxylase to activate Vitamin D
• Bone: ↑ Bone resorption → releases Ca²⁺ and PO₄³⁻ into blood (osteoclast activation)
• Gut (indirect): Via activation of Vitamin D → ↑ intestinal Ca²⁺ absorption

Net Effect of PTH

PTH increases blood calcium and decreases blood phosphate — this is the opposite of calcitonin.

PTH vs. Calcitonin — Antagonistic Pair

• PTH (Parathyroid): ↑ Blood Ca²⁺ (hypercalcemic hormone)
• Calcitonin (Thyroid — C cells): ↓ Blood Ca²⁺; promotes Ca²⁺ deposition in bone (hypocalcemic hormone)

Memory Tip

PTH = PhosphoTuriHormone — it causes phosphaturia (phosphate loss in urine) while retaining calcium.