Parathyroid Gland

NCERT grounding

Class 11 NCERT Biology, Chapter 19 Chemical Coordination and Integration, section 19.2.5 is the primary anchor. The passage is short — five sentences — but every NEET question on this gland in the last decade can be traced back to it. It states the number and location of the glands, names the hormone, identifies its chemical class, lists the three target effects, and closes with the regulator. NIOS Biology Chapter 17 echoes the same content under the name parathormone and adds that the glands are partially or wholly embedded in the thyroid.

"In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes of the thyroid gland. The parathyroid glands secrete a peptide hormone called parathyroid hormone (PTH). The secretion of PTH is regulated by the circulating levels of calcium ions." — NCERT, §19.2.5

Three nouns from this paragraph — four, peptide and calcium — are the spine of every memorisation strategy that follows. The remainder of NCERT 19.2.5 tells you what PTH does: bone resorption, renal reabsorption of Ca2+, increased Ca2+ absorption from digested food, and the descriptor hypercalcaemic. Hold those six words and you have already covered every PTH-tagged stem NEET has set from 2016 through 2024.

Anatomy and embedding in the thyroid

The parathyroid glands are four discrete bodies of glandular epithelium — two superior and two inferior — lying on the posterior (dorsal) surface of the thyroid gland. NCERT Figure 19.3 shows them as four small ovals studded on the dorsal aspect of the thyroid lobes; the ventral view of the thyroid (Figure 19.3a) does not reveal them at all. Each gland is roughly the size of a grain of rice (about 6 mm long, 3–4 mm wide) and the whole set weighs less than 200 mg — a striking contrast with the 25-g thyroid that hosts them.

Histologically the gland is built from two cell populations. Chief cells (also called principal cells) are the dominant population; these are the cells that synthesise and secrete PTH. Oxyphil cells appear after puberty, are larger and have abundant mitochondria; their precise function remains poorly defined and NCERT does not list them. For NEET purposes, parathyroid = chief cells = PTH.

The clinical importance of the embedding is real. During thyroidectomy (for thyroid cancer or large goitre) surgeons must positively identify all four parathyroids and leave them with their blood supply intact, because accidental removal produces an immediate fall in blood Ca2+ and acute tetany within 24–48 hours. NCERT does not require this clinical detail, but it explains why two glands are described as a single anatomical complex: their fates are surgically and embryologically linked.

PTH — the peptide hormone

NCERT classifies PTH explicitly as a peptide hormone. That single word answers more than it appears to. Because PTH is a peptide it is water-soluble, travels free in plasma (not bound to a carrier protein), has a short half-life of three to four minutes, and cannot cross the lipid bilayer of its target cells. Its receptor is therefore on the cell surface — the PTH1R, a G-protein-coupled receptor — and the signal is transduced inside the cell by cyclic AMP rising and activating protein kinase A. This is precisely the membrane-receptor / second-messenger pathway that NCERT contrasts with the steroid pathway in section 19.4.

The hormone in its mature form is a 84-amino-acid single chain, but only the first 34 residues at the amino-terminus are needed for full biological activity (this fragment is used as the therapeutic drug teriparatide for osteoporosis). NEET does not test the residue number — it does test that PTH is a peptide and is therefore not a steroid. NEET 2024 Q.156 used exactly this dichotomy to identify glucagon, another peptide, as the non-steroid in a four-option list. The same option-elimination logic catches PTH.

Three hypercalcaemic actions of PTH

NCERT lists three target effects of PTH, each on a different organ. Internalise them as a triangle — bone, kidney, gut — because every NEET question on this gland either asks you to confirm one effect (2022 Q.191), confirm two effects together (2018 Q.161 on osteoporosis), or pair the gland with the opposing calcitonin (2016 Q.83). The triangle below is the canonical ordering NCERT uses.

Why "resorption", not "reabsorption", at bone

Two near-identical English words encode two different processes. Bone resorption (NCERT's word) is the osteoclastic dissolution of mineralised matrix — bone is being broken down. Renal reabsorption is the recovery of a filtered solute from tubular fluid back into blood — nothing is being broken down, only retrieved. NEET 2022 Q.191 lists both as separate PTH actions on purpose: "stimulates the process of bone resorption" and "reabsorption of Ca2+ by renal tubules" are both true and both belong to PTH. Confusing the two pairs sinks the question.

The vitamin-D bridge to the gut

Of the three PTH actions, the gut effect is the one students most often miss — because PTH does not act on the intestinal cell directly. The pathway is a three-step relay. First, skin-synthesised or dietary vitamin D (cholecalciferol) is hydroxylated in the liver to 25-hydroxy-vitamin-D (calcidiol), the major storage form. Second, the kidney holds the rate-limiting enzyme 1α-hydroxylase; PTH up-regulates this enzyme, converting 25-OH-D into the active 1,25-dihydroxy-vitamin-D (calcitriol). Third, calcitriol reaches the intestinal mucosa and induces the calcium-binding protein calbindin, which drives Ca2+ uptake across the brush border.

NCERT compresses this entire relay into a single clause — "PTH … increases Ca2+ absorption from the digested food" — but NEET examiners can still test it cleanly by negating it. Option (d) of NEET 2022 Q.191 stated that PTH "decreases the absorption of Ca2+ from digested food"; that statement is wrong, and recognising it as wrong was the key to the four-correct-statement answer. The vitamin-D bridge is also the reason rickets, osteomalacia and chronic kidney disease all derange calcium balance — they break this relay at different points.

PTH vs calcitonin — the antagonistic pair

NCERT introduces calcitonin in the immediately preceding section (19.2.4) and explicitly closes the parathyroid section by saying PTH "along with TCT … plays a significant role in calcium balance". The two hormones are antagonistic, meaning they pull blood Ca2+ in opposite directions and together hold it within the narrow physiological band (≈ 9–11 mg/dL). NEET 2016 Q.83 tested exactly this — asking which of four pairs is not antagonistic — and parathormone–calcitonin was correctly listed as one of the antagonistic pairs.

Regulation by Ca2+ feedback — no pituitary tropic hormone

Most endocrine glands you have studied so far run through the hypothalamus–pituitary axis. Thyroid follows TSH, adrenal cortex follows ACTH, gonads follow FSH/LH. The parathyroid is the prominent exception. There is no PTH-stimulating hormone from the pituitary. Instead, the chief cell directly samples extracellular Ca2+ through its plasma-membrane calcium-sensing receptor (CaSR), a G-protein-coupled receptor. When plasma Ca2+ falls, CaSR is less occupied and the cell secretes PTH; when plasma Ca2+ climbs, CaSR is occupied and PTH secretion is suppressed.

This is a direct negative-feedback loop on the regulated variable itself. NCERT captures this in the line "the secretion of PTH is regulated by the circulating levels of calcium ions" — short, but examinable. NEET often probes whether students recognise that PTH is not under pituitary control; matching questions (e.g. NEET 2020 Q.63) that list hypopituitary, hyperthyroid, adrenal and pancreatic disorders never list a hypo-PTH-tropic state because no such tropic hormone exists.

Hypoparathyroidism and hyperparathyroidism

Two clinical states bracket the gland and each maps neatly to one PTH effect taken too far in the wrong direction.

Worked examples

Common confusion & NEET traps