Thyroid Hormone

Question 1

Front

Answer 1

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Question 2

What is the first step in thyroid hormone synthesis?

Answer 2

Iodide trapping: Thyroid follicular cells actively transport iodide (I⁻) from blood into the cell via the Na⁺/I⁻ symporter (NIS).

Question 3

How is thyroglobulin synthesized?

Answer 3

Thyroglobulin (Tg) is produced by follicular cells, exported into the follicular lumen, and serves as a scaffold for hormone synthesis.

Question 4

What enzyme oxidizes iodide to iodine?

Answer 4

Thyroid peroxidase (TPO) converts I⁻ to reactive iodine (I₂) at the follicular cell-lumen interface.

Question 5

How are T3 and T4 formed?

Answer 5

1. Iodination: Iodine binds to tyrosine residues on Tg → MIT (monoiodotyrosine) and DIT (diiodotyrosine). 2. Coupling: TPO links MIT + DIT → T3; DIT + DIT → T4.

Question 6

What % of secreted thyroid hormone is T4 vs T3?

Answer 6

T4 (90%, inactive prohormone); T3 (10%, biologically active). Most T3 is generated peripherally (liver/kidneys) via deiodination of T4.

Question 7

What regulates TSH release from the pituitary?

Answer 7

TRH (from hypothalamus) stimulates TSH; T3/T4 inhibit TSH/TRH via negative feedback.

Question 8

What is the role of TSH in thyroid function?

Answer 8

TSH binds to receptors on follicular cells → ↑ iodide uptake, Tg synthesis, TPO activity, and T3/T4 release.

Question 9

What are the clinical features of hypothyroidism?

Answer 9

Fatigue, weight gain, cold intolerance, bradycardia, myxedema (non-pitting edema), dry skin, constipation, depression.

Question 10

What causes primary vs secondary hypothyroidism?

Answer 10

Primary: Thyroid failure (Hashimoto’s, iodine deficiency). Secondary: Pituitary TSH deficiency (rare).

Question 11

What is myxedema coma?

Answer 11

Life-threatening severe hypothyroidism: Hypothermia, hypotension, coma. Treat with IV levothyroxine + glucocorticoids.

Question 12

What are the clinical features of hyperthyroidism?

Answer 12

Weight loss, heat intolerance, tachycardia, tremors, anxiety, exophthalmos (Graves’), ↑ appetite, diarrhea.

Question 13

What causes Graves’ disease?

Answer 13

Autoantibodies (TSI) mimic TSH → overstimulate thyroid → ↑ T3/T4. Associated with ophthalmopathy and dermopathy.

Question 14

What is thyrotoxicosis?

Answer 14

Clinical syndrome of excess thyroid hormone (from Graves’, toxic nodules, or thyroiditis). Labs: ↓ TSH, ↑ T3/T4.

Question 15

How does TRH stimulation test differentiate hypo causes?

Answer 15

Primary hypo: High TSH (no response to TRH). Secondary (pituitary): Low TSH (no rise). Tertiary (hypothalamic): Delayed TSH rise.

Question 16

What drugs inhibit thyroid hormone synthesis?

Answer 16

1. Thioamides (methimazole, PTU) block TPO. 2. Perchlorate inhibits NIS. 3. Lithium inhibits iodide organification.