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Thyrocalcitonin (TCT) and Blood Calcium

Biology · Chemical Coordination and Integration · NEET

Thyrocalcitonin (TCT), also called calcitonin, is a protein (peptide) hormone made by the thyroid gland. Its one job you must remember for NEET is that it LOWERS the calcium level in the blood. Memory hook: TCT "Takes Calcium away Together" from the blood and locks it into the bones, the exact opposite of PTH.
Blood Calcium Control: TCT vs PTHBlood Ca levelkept steadyTHYROID glandreleases TCTPARATHYROIDreleases PTHLOWERS Ca(when Ca is high)RAISES Ca(when Ca is low)Antagonistic pairTCT = protein hormone from thyroid = hypocalcemic
Thyrocalcitonin (TCT) from the thyroid lowers blood calcium, while PTH from the parathyroid raises it. The two hormones are antagonistic and keep blood calcium steady.

Your doubts, answered

Does thyrocalcitonin increase or decrease blood calcium?

It DECREASES blood calcium. When calcium in the blood is too high, the thyroid releases TCT. TCT stops bone from breaking down and pushes calcium out of the blood into the bones. So it is a hypocalcemic hormone (hypo = low). NCERT says clearly: 'thyrocalcitonin (TCT) which regulates the blood calcium levels.'

Which gland makes thyrocalcitonin, the thyroid or the parathyroid?

The THYROID gland makes thyrocalcitonin. This is a very common trap. Because the hormone controls calcium, students wrongly link it to the parathyroid. But NCERT places TCT under the thyroid gland. The parathyroid makes a different hormone called PTH (parathyroid hormone), which raises calcium.

Is thyrocalcitonin a protein hormone or a steroid hormone?

It is a PROTEIN (peptide) hormone. NCERT calls it 'a protein hormone called thyrocalcitonin.' This matters because peptide hormones act on the cell surface (membrane receptors), while steroid hormones enter the cell. TCT is not a steroid and is not made from cholesterol.

How is TCT different from PTH?

They are antagonistic (opposite). TCT lowers blood calcium; PTH raises blood calcium. TCT is from the thyroid; PTH is from the parathyroid. When calcium is high, TCT works. When calcium is low, PTH works. Together they keep calcium steady, which is called calcium homeostasis.

Why is calcitonin called a hypocalcemic hormone?

'Hypocalcemic' means it brings calcium DOWN. Calcitonin blocks bone resorption (bone breakdown) and reduces calcium release from bone into blood. So the blood calcium level falls. Remember: calciTONIN 'TONES DOWN' calcium.

⚠️ The NEET trap
✗ Thyrocalcitonin is secreted by the parathyroid gland and raises blood calcium.
✓ Thyrocalcitonin (TCT) is secreted by the THYROID gland and LOWERS blood calcium. The parathyroid gland secretes PTH, which raises calcium.
🧠 Read the gland name carefully: 'thyro' in thyrocalcitonin means THYROID, not parathyroid. NTA loves this source-swap in match-the-column questions.

Real NEET questions

NEET 2023

Which of the following statements are correct with respect to the hormone and its function? (A) Thyrocalcitonin (TCT) regulates the blood calcium level. (B) In males, FSH and androgens regulate spermatogenesis. (C) Hyperthyroidism can lead to goitre. (D) Glucocorticoids are secreted in Adrenal Medulla. (E) Parathyroid hormone is regulated by circulating levels of sodium ions. Choose the most appropriate answer from the options given below:

A · (B) and (C) only
B · (A) and (D) only
C · (C) and (E) only
D · (A) and (B) only ✓
Solution: (A) is correct: TCT regulates blood calcium. (B) is correct: FSH and androgens regulate spermatogenesis in males. (C) is wrong: hyperthyroidism does not cause goitre. (D) is wrong: glucocorticoids come from the adrenal CORTEX, not medulla. (E) is wrong: PTH is regulated by calcium, not sodium. So (A) and (B) only.
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: Parathormone (raises Ca) and Calcitonin (lowers Ca) ARE antagonistic. Insulin and Glucagon are antagonistic. Aldosterone and ANF are antagonistic for blood pressure. Relaxin and Inhibin act on unrelated targets, so they are NOT antagonistic. Answer: Relaxin - Inhibin.
NEET 2023

Given below are two statements: 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. In the light of the above statements, 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 stimulates bone resorption. Statement II is false: PTH and TCT control CALCIUM balance, not carbohydrate metabolism. So only Statement I is correct.

Solved Chemical Coordination and Integration NEET PYQs

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Frequently asked

What is the full form of TCT?

TCT stands for ThyroCalciToniN, also simply called calcitonin. It is a protein hormone from the thyroid gland that lowers blood calcium.

Is TCT the same as calcitonin?

Yes. Thyrocalcitonin and calcitonin are two names for the same hormone. NEET can use either name, so know both.

When is TCT released?

TCT is released when the blood calcium level rises above normal. It then brings the calcium level back down, mainly by acting on bone.

What happens if there is too little TCT?

For NEET, the key learning is the normal role of TCT in lowering calcium. In humans its effect is mild compared to PTH, but its exam job is always 'lowers blood calcium.'

Is thyrocalcitonin a NEET-important hormone?

Yes. It appears in match-the-hormone, antagonistic-pair, and statement-based questions. The two facts to lock: source = thyroid, action = lowers blood calcium.

Related concepts:
Thyroid Gland: Thyroxine (T3 and T4)Parathyroid Gland: PTH and Bone ResorptionPTH vs Calcitonin: Calcium RegulationOsteoporosis: Causes and Hormones InvolvedPeptide vs Steroid Hormones
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Chemical Coordination and Integration PYQs