🎓 Prepared by students from Boğaziçi University

What is the Thyroid Gland?

The thyroid gland is a butterfly-shaped endocrine organ in the neck that produces hormones controlling the body's metabolic rate. Its two lobes are packed with follicles that trap iodine and synthesize thyroxine (T4) and triiodothyronine (T3).

Short answer

The thyroid gland consists of follicular cells that produce T3 and T4 under TSH stimulation, and parafollicular (C) cells that secrete calcitonin to help lower blood calcium.

Thyroid Hormone Synthesis

1
Hypothalamus
Releases TRH (thyrotropin-releasing hormone)
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2
Pituitary gland
TRH stimulates release of TSH (thyroid-stimulating hormone)
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3
Follicular cells
TSH triggers iodine trapping and thyroglobulin synthesis
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4
Iodination
Iodine attaches to tyrosine residues on thyroglobulin
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5
Hormone release
T4 and T3 are cleaved from thyroglobulin and released into blood

Step-by-step worked examples

The healthy adult thyroid gland weighs about how much, and how many lobes does it have?

Normal weight ≈ 15–20 g
Two lateral lobes connected by the isthmus
Sits anterior to the trachea, below the larynx

The thyroid secretes T4 and T3 in about a 20:1 ratio. If total daily hormone output equals 105 relative units, how many units are T4 vs T3?

Ratio T4:T3 ≈ 20:1, so 21 parts total
105 ÷ 21 = 5 units per part
T4 = 20 × 5 = 100 units; T3 = 1 × 5 = 5 units

Recommended daily iodine intake for adults is about 150 mcg. If someone consumes 3 servings of iodized salt each providing 40 mcg, do they meet the requirement?

3 × 40 mcg = 120 mcg
120 mcg < 150 mcg recommended
They fall short by 30 mcg/day

Flashcards

Quick quiz

Q1.Which hormone is produced in the largest quantity by the thyroid?

Correct answer: B. The thyroid secretes far more T4 than T3, roughly a 20:1 ratio; T4 is converted to the more active T3 peripherally.

Q2.What stimulates the thyroid follicular cells to release hormone?

Correct answer: B. TSH from the pituitary binds follicular cell receptors, triggering hormone release.

Q3.What do parafollicular (C) cells produce?

Correct answer: C. C cells sit between follicles and secrete calcitonin, which lowers blood calcium.

Q4.Which structure joins the thyroid's two lobes?

Correct answer: A. The isthmus is the thin bridge of thyroid tissue anterior to the trachea connecting the lobes.

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Common mistakes

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The thyroid produces mostly T3. — Correct: The thyroid secretes mostly T4; peripheral tissues convert it to the more active T3.

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TSH is made by the thyroid gland. — Correct: TSH is made by the anterior pituitary and acts ON the thyroid gland.

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Calcitonin comes from the parathyroid gland. — Correct: Calcitonin comes from the thyroid's C cells; the parathyroid glands produce PTH instead.

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Iodine has no role in thyroid hormone production. — Correct: Iodine is essential — it is incorporated into thyroglobulin to synthesize T3 and T4.

FAQ

What is the thyroid gland?

A butterfly-shaped endocrine gland in the neck that produces T3 and T4 to regulate metabolism, plus calcitonin from C cells.

What is the thyroid hormone formula/pathway?

TRH (hypothalamus) → TSH (pituitary) → T3/T4 (thyroid follicular cells), a classic negative-feedback axis.

What are examples of thyroid gland function in the body?

Regulating heart rate, body temperature, and metabolic rate are everyday examples of thyroid hormone activity.

How is thyroid gland structure examined clinically?

Palpation, ultrasound, and blood tests for TSH, T3, and T4 are common ways to assess thyroid structure and function.