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PTH vs Calcitonin: How the Body Controls Blood Calcium

Biology · Chemical Coordination and Integration · NEET

PTH (parathyroid hormone) raises blood calcium, and calcitonin (TCT) lowers blood calcium. They are antagonistic hormones that work in opposite directions to keep your blood Ca2+ level steady. Memory hook: "PTH Pulls calcium UP from bone, Calcitonin Calms it DOWN into bone."
Blood Calcium Control: PTH vs CalcitoninBlood Ca2+kept steadyPTHParathyroid glandRAISES calcium(hypercalcemic)Calcitonin (TCT)Thyroid glandLOWERS calcium(hypocalcemic)Bone resorption + kidney reabsorb + gut absorbDeposits Ca2+ back into boneAntagonistic pair, like Insulin vs Glucagon
PTH from the parathyroid raises blood calcium, while calcitonin (TCT) from the thyroid lowers it. This antagonistic pair keeps blood Ca2+ balanced.

Your doubts, answered

Does PTH increase or decrease blood calcium?

PTH INCREASES blood calcium. It is a hypercalcemic hormone (hyper = high). PTH does three things to raise Ca2+: it stimulates bone resorption (dissolving bone to release calcium), it makes the kidney tubules reabsorb more Ca2+, and it increases Ca2+ absorption from the digested food in the gut. Calcitonin does the opposite and lowers blood calcium.

Is calcitonin the same as thyrocalcitonin (TCT)?

Yes. Calcitonin and thyrocalcitonin (TCT) are the same hormone. NCERT calls it thyrocalcitonin (TCT) because it is secreted by the thyroid gland. Do not confuse it with thyroxine (T3/T4). TCT only controls blood calcium; thyroxine controls the basal metabolic rate.

Which gland makes PTH and which makes calcitonin?

PTH is made by the PARATHYROID glands (four small glands on the back of the thyroid). Calcitonin (TCT) is made by the THYROID gland itself. This is a common trap: both hormones control calcium, but they come from two different glands sitting next to each other.

What is bone resorption?

Bone resorption means the dissolving or demineralisation of bone. PTH stimulates this process so that calcium stored in the bone is released into the blood. This is why long-term high PTH weakens bones. Note the spelling: it is reSORPTION (taking calcium out of bone), not absorption.

Are PTH and calcitonin antagonistic?

Yes. PTH and calcitonin are antagonistic hormones, meaning they have opposite effects. PTH raises blood Ca2+ and calcitonin lowers it. This antagonistic pair keeps calcium balanced, similar to how insulin and glucagon act as an antagonistic pair for blood glucose.

What stimulates the parathyroid gland to release PTH?

A FALL in blood calcium (Ca2+) levels stimulates PTH release. The secretion of PTH is regulated by the circulating levels of calcium ions, NOT by sodium ions, not by vitamin D directly, and not by bone calcium. When blood Ca2+ drops, PTH is released to bring it back up.

⚠️ The NEET trap
✗ PTH secretion is controlled by the level of sodium ions, and calcitonin is made by the parathyroid gland.
✓ PTH secretion is controlled by the circulating level of CALCIUM ions, and calcitonin (TCT) is made by the THYROID gland (not parathyroid).
🧠 NEET swaps the ion (Ca2+ not Na+) and swaps the gland. PTH = parathyroid, Calcitonin/TCT = thyroid. Read the gland and the ion carefully.

Real NEET questions

NEET 2016

Which of the following pairs of hormones are not antagonistic (having opposite effects) to each other?

A · Parathormone - Calcitonin
B · Insulin - Glucagon
C · Aldosterone - Atrial Natriuretic Factor
D · Relaxin - Inhibin ✓
Solution: Antagonistic pairs have opposite effects. Parathormone (PTH) raises blood Ca2+ while Calcitonin lowers it, so this pair IS antagonistic. Insulin lowers and Glucagon raises blood glucose (antagonistic). Aldosterone raises and ANF lowers blood pressure (antagonistic). Relaxin (relaxes pelvic ligaments) and Inhibin (inhibits FSH) act on unrelated targets and are NOT antagonistic. So the exception is Relaxin - Inhibin (D).
NEET 2023

Statement I: Parathyroid hormone acts on bones and stimulates the process of bone resorption. Statement II: Parathyroid hormone along with Thyrocalcitonin plays a significant role in carbohydrate metabolism. Choose the correct answer.

A · Both Statement I and Statement II are true
B · Both Statement I and Statement II are false
C · Statement I is correct but Statement II is false ✓
D · Statement I is incorrect but Statement II is true
Solution: Statement I is correct: PTH acts on bones and stimulates bone resorption (dissolution/demineralisation). Statement II is false: PTH along with TCT plays a significant role in CALCIUM balance, not carbohydrate metabolism. So only Statement I is correct (C).

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Frequently asked

Is PTH a peptide or steroid hormone?

PTH is a peptide hormone secreted by the parathyroid glands. Calcitonin (TCT) is also a peptide hormone from the thyroid.

What happens if PTH is oversecreted (hyperparathyroidism)?

Too much PTH causes too much bone resorption, so bones become weak and blood calcium rises abnormally. This can lead to bone deformities and soft, brittle bones.

Do PTH and calcitonin together control calcium balance?

Yes. NCERT states that along with TCT, PTH plays a significant role in calcium balance in the body. PTH pushes calcium up and calcitonin pulls it down, keeping the level steady.

Does PTH act on the kidney?

Yes. PTH stimulates the renal tubules to reabsorb Ca2+ back into the blood, and it also helps activate vitamin D which increases calcium absorption from the gut. Both actions raise blood calcium.

Which hormone is hypercalcemic and which is hypocalcemic?

PTH is hypercalcemic (raises blood Ca2+). Calcitonin (TCT) is hypocalcemic (lowers blood Ca2+). Remembering these two labels answers most NEET questions on calcium hormones.

Related concepts:
Parathyroid Gland: PTH and Bone ResorptionThyrocalcitonin (TCT) and Blood CalciumOsteoporosis: Causes and Hormones InvolvedInsulin vs Glucagon: Antagonistic ActionThyroid Gland: Thyroxine (T3 and T4)
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