Biology · Chemical Coordination and Integration · NEET
Both are made by the follicular cells of the thyroid using iodine. T4 is thyroxine (tetraiodothyronine) and carries 4 iodine atoms. T3 is triiodothyronine and carries 3 iodine atoms. The number after T tells you the iodine count. For NEET you mainly need to know both are thyroid hormones that raise basal metabolic rate.
Iodine is a raw material joined into the hormone molecule, so without enough iodine the thyroid cannot make T3 and T4 at a normal rate. If iodine is low in the diet, hormone output falls (hypothyroidism) and the gland enlarges. This enlarged thyroid is called goitre. This is why iodised salt is used to prevent goitre.
Thyroxine is NOT a protein hormone. NCERT places thyroid hormones (T3 and T4) in a special group called iodothyronines, which are derived from the amino acid tyrosine plus iodine. So they are amino-acid based, not peptide or protein hormones. Do not confuse them with thyrocalcitonin (TCT), which the thyroid also makes but is a protein hormone.
The isthmus is a thin flap of connective tissue that connects the two lobes of the thyroid gland. It is a structural link only, not a hormone-making part. NEET diagram-based questions may ask you to label the isthmus between the two lobes, so remember: two lobes on either side of the trachea, joined by the isthmus.
No. This is a common trap. The sleep-wake (diurnal) rhythm is controlled by melatonin from the pineal gland, not by thyroid hormones. Thyroid hormones control basal metabolic rate, red blood cell formation, metabolism of carbohydrates, proteins and fats, and water and electrolyte balance. NEET 2023 used exactly this trap.
Which of the following are NOT under the control of thyroid hormone? A. Maintenance of water and electrolyte balance, B. Regulation of basal metabolic rate, C. Normal rhythm of sleep-wake cycle, D. Development of immune system, E. Support the process of R.B.Cs formation
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The follicular cells secrete thyroxine (T4) and triiodothyronine (T3), both made using iodine. The thyroid also secretes a protein hormone called thyrocalcitonin (TCT), which lowers blood calcium.
The anterior pituitary releases TSH (thyroid stimulating hormone), which stimulates the thyroid to synthesise and secrete T3 and T4. So the pituitary sits above the thyroid in the control chain.
Iodine deficiency in the diet lowers thyroid hormone synthesis. This causes hypothyroidism and enlargement of the thyroid gland, known as goitre.
Thyroxine is T4 (tetraiodothyronine), with 4 iodine atoms. T3 (triiodothyronine) is the other thyroid hormone, with 3 iodine atoms.
