CDS (I) 2023 General Knowledge · Question 40 of 120

Correct — D, Iodine. Thyroxin is an iodinated hormone: the thyroid gland takes up iodide from the blood and attaches iodine atoms to the amino acid tyrosine to build the molecule, which is why the hormone cannot be made at all if the diet supplies no iodine. NCERT's Class 10 text states it in as many words — 'Iodine is necessary for the thyroid gland to make thyroxin hormone. Thyroxin regulates carbohydrate, protein and fat metabolism in the body so as to provide the best balance for growth. Iodine is essential for the synthesis of thyroxin. In case iodine is deficient in our diet, there is a possibility that we might suffer from goitre.' That last consequence is the reason ordinary salt sold in India is iodised, and the reason the swollen neck of goitre is the classic sign of the deficiency. Note the trap built into the options: sodium chloride is common salt, and a candidate who half-remembers 'salt' as the answer will reach for it. The salt is only the carrier; the iodine added to it is what the gland needs.

• (a)NaCl — Common salt, and the designed trap. Iodised salt prevents goitre because iodine is added to it, not because sodium chloride itself has any part in making the hormone.
• (b)KCl — Potassium chloride is used as a salt substitute and in medicine, but potassium has no role in thyroxin synthesis.
• (c)Cholesterol — Cholesterol is the precursor of the steroid hormones — cortisol, aldosterone, oestrogen, testosterone. Thyroxin is not a steroid; it is built from tyrosine and iodine.

The thyroid gland sits in the neck and secretes thyroxin, which sets the rate at which the body burns carbohydrate, protein and fat, and so governs growth and general metabolic rate. The hormone is made by iodinating the amino acid tyrosine, so dietary iodine is a structural requirement rather than a helper. Where soil and water are poor in iodine, the gland enlarges in the attempt to capture more of it, producing the visible swelling called goitre.

Hormone questions are best held by their chemical family, because that single fact answers most of what is asked. Thyroxin is an amino-acid derivative carrying iodine; the adrenal and sex hormones are steroids made from cholesterol; insulin and growth hormone are proteins. Sorting the four options by family disposes of cholesterol at once and leaves only the question of which ion matters, and the answer is in the name of the salt on every Indian kitchen shelf. The public-health note worth carrying is that universal salt iodisation is the reason endemic goitre has retreated in the sub-Himalayan belt where it was once common.

• Iodine is essential for the synthesis of thyroxin; the hormone is an iodinated derivative of the amino acid tyrosine.
• Thyroxin regulates carbohydrate, protein and fat metabolism, and so sets the body's metabolic rate.
• Iodine deficiency in the diet causes goitre, whose visible sign is a swollen neck.
• Iodised salt is common salt with iodine added; sodium chloride itself plays no part in making the hormone.
• Steroid hormones such as cortisol and the sex hormones are made from cholesterol, which is why cholesterol is a plausible-looking but wrong option here.

• Choosing common salt because iodised salt prevents goitre; the active ingredient is the added iodine.
• Assigning thyroxin to the steroid family and reaching for cholesterol.
• Confusing the thyroid with the parathyroid, which regulates calcium and phosphate rather than metabolic rate.

As a which-element item on thyroxin, as a hormone-to-gland match, or as a deficiency-disease item on goitre.

• practice — not a real PYQ

  A deficiency of iodine in the diet leads most directly to which one of the following conditions?

  • (a)Rickets
  • (b)Goitre
  • (c)Scurvy
  • (d)Beriberi

  Answer(b) Goitre — the thyroid enlarges when it cannot obtain enough iodine to make thyroxin, producing the characteristic swelling of the neck.

• practice — not a real PYQ

  Which one of the following hormones is derived from cholesterol?

  • (a)Thyroxin
  • (b)Insulin
  • (c)Cortisol
  • (d)Growth hormone

  Answer(c) Cortisol — a steroid hormone of the adrenal cortex. Thyroxin is built from tyrosine and iodine, while insulin and growth hormone are proteins.