Biology · Chemical Coordination and Integration · NEET
The parathyroid gland secretes Parathyroid Hormone (PTH), a peptide hormone that RAISES blood calcium. So PTH is a hypercalcemic hormone. It does three things: it acts on bones and stimulates bone resorption (dissolving/demineralising bone to release Ca2+), it increases Ca2+ reabsorption by the kidney tubules, and it increases Ca2+ absorption from digested food. Memory hook: PTH = "Pull To High" calcium (P-T-H pulls calcium up in blood).
When blood calcium falls, the parathyroid gland releases PTH, which raises Ca2+ through three routes: bone resorption, kidney reabsorption and gut absorption. PTH is a hypercalcemic hormone.
Your doubts, answered
Does PTH increase or decrease blood calcium?
PTH INCREASES blood calcium. It is a hypercalcemic hormone (hyper = high). When blood Ca2+ falls too low, the parathyroid gland releases PTH to pull calcium back up. Do not confuse it with thyrocalcitonin (TCT), which LOWERS blood calcium. PTH up, TCT down.
What exactly is bone resorption?
Bone resorption means the dissolving or demineralising of bone. PTH acts on bone and breaks down the mineral matrix, releasing stored Ca2+ into the blood. Think of the bone as a calcium bank: when blood calcium is low, PTH withdraws calcium from the bone bank. NCERT uses the exact words dissolution and demineralisation for this.
What signal makes the parathyroid gland release PTH?
A FALL in blood Ca2+ levels is the stimulus. NCERT says PTH secretion is regulated by the circulating levels of calcium ions. Low blood calcium turns PTH on; the stimulus is NOT low bone calcium and NOT vitamin D. This exact point was asked in NEET 2019.
How many parathyroid glands do humans have and where are they?
Humans have FOUR parathyroid glands, arranged as two pairs. They sit on the back (posterior) side of the thyroid gland, one pair in each of the two thyroid lobes. A common exam point: they are behind the thyroid but are a separate gland with the opposite calcium effect.
Is PTH a peptide hormone or a steroid hormone?
PTH is a PEPTIDE hormone (protein-type), so it acts through membrane receptors, not by entering the cell. This matters because peptide hormones cannot cross the cell membrane the way steroid hormones do. NCERT explicitly calls PTH a peptide hormone.
Does PTH control carbohydrate metabolism?
No. This is a classic trap. PTH controls CALCIUM balance only (bone, kidney, gut). It does NOT regulate carbohydrate metabolism. NEET 2023 tested exactly this: the statement that PTH along with thyrocalcitonin plays a role in carbohydrate metabolism is FALSE.
⚠️ The NEET trap ✗ PTH lowers blood calcium by pushing calcium into bones (like calcitonin), and it also helps carbohydrate metabolism. ✓ PTH RAISES blood calcium (hypercalcemic) by bone resorption, renal reabsorption and gut absorption. It has NOTHING to do with carbohydrate metabolism. 🧠 NTA loves swapping PTH and TCT, and loves adding a fake 'carbohydrate metabolism' line. Lock it: PTH = calcium UP, calcium job ONLY.
Real NEET questions
NEET 2019
Which of the following conditions will stimulate parathyroid gland to release parathyroid hormone?
A · Fall in active Vitamin D levels
B · Fall in blood Ca+2 levels ✓
C · Fall in bone Ca+2 levels
D · Rise in blood Ca+2 levels
Solution: PTH secretion is regulated by circulating calcium ion levels. A FALL in blood Ca2+ is the direct stimulus that makes the parathyroid glands release PTH, which then raises blood Ca2+ by bone resorption, renal reabsorption and gut absorption. The trigger is low blood calcium, not bone calcium or vitamin D.
NEET 2022
Which of the following are NOT the effects of Parathyroid hormone? (a) Stimulates the process of bone resorption, (b) Decreases Ca2+ level in blood, (c) Reabsorption of Ca2+ by renal tubules, (d) Decreases the absorption of Ca2+ from digested food, (e) Increases metabolism of carbohydrates.
A · (a) and (c) only
B · (b), (d) and (e) only ✓
C · (a) and (e) only
D · (b) and (c) only
Solution: PTH is hypercalcemic: it stimulates bone resorption (a, true), causes Ca2+ reabsorption by renal tubules (c, true) and INCREASES Ca2+ absorption from food. So the false statements are (b) it decreases blood Ca2+, (d) it decreases gut absorption, and (e) it increases carbohydrate metabolism. Hence (b), (d) and (e) are NOT effects of PTH.
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 bone and stimulates bone resorption (demineralisation) to raise blood Ca2+. Statement II is false: PTH controls calcium balance, not carbohydrate metabolism. So Statement I is correct but Statement II is false.
Solved Chemical Coordination and Integration NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
PTH is a peptide hormone secreted by the four parathyroid glands. Being a peptide, it acts through membrane receptors on target cells rather than entering the cell.
Why is PTH called a hypercalcemic hormone?
Because it raises (hyper) blood calcium (calcemic). It pulls Ca2+ up in the blood through bone resorption, kidney reabsorption and increased gut absorption of calcium.
What is the difference between PTH and thyrocalcitonin?
PTH raises blood calcium (hypercalcemic) while thyrocalcitonin (TCT) from the thyroid lowers blood calcium (hypocalcemic). They are antagonistic and together keep blood Ca2+ balanced.
Does PTH need vitamin D to work?
Vitamin D helps calcium absorption from the gut, but the direct trigger for PTH release is a fall in blood Ca2+, not vitamin D levels. NEET 2019 specifically rejected 'fall in vitamin D' as the stimulus.
How does PTH affect the kidney?
PTH stimulates the renal tubules to reabsorb more Ca2+, so less calcium is lost in urine and blood calcium rises. This is one of its three calcium-raising actions.
