MPSC 2026 Prelims Paper-I · Question 62 of 100

Correct — option (3), Thyroid. The word 'parafollicular' settles the question on its own. The thyroid is the one gland in the body that is built out of follicles: hollow spherical sacs whose lining cells (the follicular cells) trap iodine and manufacture the thyroid hormones thyroxine (T4) and tri-iodothyronine (T3). Scattered between and around those follicles is a second, quite separate cell population — the parafollicular cells, also called C cells, 'para-' meaning beside the follicle. These C cells secrete calcitonin, a polypeptide hormone of 32 amino acids. Calcitonin lowers the level of calcium in the blood: it damps down the bone-dissolving activity of osteoclasts, so less calcium is released from bone into the circulation, and it reduces the reabsorption of calcium in the kidney tubules, so more is lost in urine. It is therefore the functional opposite of parathyroid hormone (PTH), secreted by the four parathyroid glands sitting on the back of the thyroid, which raises blood calcium. The gland is anatomically one thing with two hormone-making jobs, and a candidate who remembers only 'thyroid = thyroxine' misses the second. Note also that the thyroid gland is a bilobed endocrine gland lying on either side of the trachea, the two lobes joined by a connecting band called the isthmus.

• (1)Salivary — The salivary glands are exocrine, not endocrine: they pour their secretion through ducts into the mouth rather than into the blood. Humans have three pairs — parotid, sub-maxillary/sub-mandibular and sub-lingual — and what they secrete is saliva, containing the starch-splitting enzyme salivary amylase (ptyalin) along with lysozyme, mucus and electrolytes. There is no follicular architecture in a salivary gland and therefore nothing that could be called a parafollicular cell, and salivary tissue produces no hormone that circulates in the blood.
• (2)Sweat — Sweat glands are exocrine glands of the skin, embedded in the dermis and opening at the body surface through a pore. Their secretion is a watery fluid whose evaporation cools the body, and which also carries away small amounts of salt, urea and lactate. Like the salivary glands they are ducted, they have no follicles, and they release nothing into the bloodstream. Sweat glands appear in the syllabus under thermoregulation and skin structure, never under the endocrine system.
• (4)Pancreas — This is the trap worth taking seriously, because the pancreas genuinely is a hormone factory and its hormones genuinely are polypeptides. It is a composite gland: the exocrine part secretes pancreatic juice, and the endocrine part — the islets of Langerhans — secretes insulin from the beta cells and glucagon from the alpha cells, both peptides. But the islet is not a follicle, so the pancreas has no parafollicular cells; and the pancreatic hormones regulate blood glucose, not blood calcium. Two words in the stem, 'parafollicular' and 'Calcitonin', each independently rule the pancreas out.

Blood calcium is held inside a narrow range by a push-pull pair of hormones, and the syllabus expects you to hold both halves. Parathyroid hormone (PTH, parathormone) comes from the four parathyroid glands on the posterior surface of the thyroid; it is hypercalcaemic, raising blood calcium by promoting the dissolution of bone matrix by osteoclasts, by increasing calcium reabsorption in the kidney tubules and by increasing absorption of calcium from the food in the gut. Calcitonin comes from the thyroid's parafollicular cells; it is hypocalcaemic, pulling blood calcium down by exactly the opposite routes. Calcium matters far beyond the skeleton — it is required for muscle contraction, for the transmission of the nerve impulse at the synapse and for blood clotting — so this pair is the body's guarantee that those processes keep working whatever the diet supplies on a given day.

The thyroid also carries the more familiar half of its workload. Its follicular cells use iodine to build thyroxine, which sets the basal metabolic rate, supports normal growth and development, and influences the metabolism of carbohydrates, proteins and fats as well as the production of red blood cells. Because iodine is essential to the molecule, dietary iodine deficiency enlarges the gland — the condition called simple or endemic goitre — and is the reason India runs a universal salt-iodisation programme. Under-secretion of thyroid hormones in a developing child causes cretinism, with stunted growth and impaired mental development; in an adult it produces the sluggish, low-metabolic-rate state called hypothyroidism. Over-secretion produces hyperthyroidism. Calcitonin sits inside this same organ but belongs to an entirely different regulatory circuit, and examiners like that disjunction.

• Calcitonin (CT), also written thyrocalcitonin (TCT), is a polypeptide hormone of 32 amino acids secreted by the parafollicular cells, or C cells, of the thyroid gland. It is hypocalcaemic — it lowers the level of calcium in the blood.
• It works by inhibiting bone resorption by osteoclasts, so less calcium leaves the skeleton for the blood, and by reducing the reabsorption of calcium in the kidney tubules, so more calcium is excreted in the urine.
• Parathyroid hormone (parathormone, PTH) from the four parathyroid glands on the back of the thyroid is its antagonist — hypercalcaemic, raising blood calcium from bone, kidney and gut. Calcitonin and PTH together hold blood calcium steady.
• The thyroid is a bilobed gland on either side of the trachea, the lobes connected by an isthmus. Its follicular cells make the iodine-containing hormones thyroxine (T4) and tri-iodothyronine (T3), which set the basal metabolic rate.
• Dietary iodine deficiency enlarges the thyroid — simple or endemic goitre. Deficiency of thyroid hormone in a growing child causes cretinism; in an adult, hypothyroidism. Calcium itself is needed for muscle contraction, nerve impulse transmission and blood clotting.

• Reading 'thyroid' as 'thyroxine only'. The thyroid makes two unrelated kinds of hormone from two different cell populations — follicular cells for T3/T4, parafollicular cells for calcitonin.
• Confusing thyroid with parathyroid because the names and the locations are almost the same. The parathyroids sit on the back of the thyroid, but their hormone raises blood calcium while the thyroid's calcitonin lowers it.
• Assuming 'polypeptide hormone' points to the pancreas. Insulin and glucagon are polypeptides, but so is calcitonin; the discriminating word in this stem is 'parafollicular', not 'polypeptide'.

Calcium regulation is asked from either end. The commission-style question gives you the cell type or the gland and asks for the hormone, as here; the UPSC-style question gives you the job — 'which hormone regulates blood calcium and phosphate' — and asks for the source. Either way the safe preparation is a two-column table of gland, cell type, hormone, and effect, learnt in both directions. Watch also for the iodine link, which is asked separately and often ('which hormone contains iodine'), and for the goitre-cretinism-hypothyroidism cluster, which is asked as a public-health item as much as a biology one.

• practice — not a real PYQ

  Which one of the following pairs of hormones acts antagonistically to keep the level of calcium in human blood within a narrow range?

  • (a)Insulin and glucagon
  • (b)Calcitonin and parathormone
  • (c)Thyroxine and adrenaline
  • (d)Aldosterone and cortisol

  Answer(b) Calcitonin and parathormone — calcitonin from the parafollicular cells of the thyroid lowers blood calcium by suppressing bone resorption and renal reabsorption, while parathormone from the parathyroid glands raises it by acting on bone, kidney and intestine. Insulin and glucagon are the antagonistic pair for blood glucose, not calcium.

• practice — not a real PYQ

  The follicular cells of the human thyroid gland require which one of the following elements to synthesise their hormones?

  • (a)Iron
  • (b)Iodine
  • (c)Zinc
  • (d)Cobalt

  Answer(b) Iodine — thyroxine (T4) and tri-iodothyronine (T3) are iodinated hormones, which is why dietary iodine deficiency enlarges the gland as simple goitre and why India iodises table salt. Cobalt belongs to vitamin B12, iron to haemoglobin.