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Thyroid Gland — Detailed Explanation (English)

The thyroid gland is one of the most important endocrine glands in the human body. It regulates metabolism, growth, body temperature, calcium balance, and influences the function of many vital organs.

📍 Location

Situated in the anterior (front) part of the neck.

Lies just below the larynx (voice box) and in front of the trachea (windpipe).

Two lobes (right and left) are connected by a central band called the isthmus.

🧠 Shape, Size & Blood Supply

Butterfly-shaped gland.

Length: about 5 cm, Width: 3 cm, Thickness: 1.5–2 cm.

Weight: approx 20–30 grams in adults.

It has an extremely rich blood supply, making it highly vascular.

Arteries:

Superior thyroid artery

Inferior thyroid artery
(Sometimes a thyroid ima artery may also be present)

🧬 Histological Structure

Functional unit: Thyroid follicles.

Follicles are spherical sacs filled with a gel-like substance called colloid, which stores a protein named thyroglobulin.

Cell types:

Follicular cells → produce T3 and T4 hormones

Parafollicular cells (C cells) → produce Calcitonin

⚗ Thyroid Hormones
Hormone Composition Function
T4 (Thyroxine) 4 iodine atoms Main circulating hormone, regulates metabolism
T3 (Tri-iodothyronine) 3 iodine atoms Most active hormone (more potent than T4)
Calcitonin Peptide hormone Lowers blood calcium level
⚙ Hormone Synthesis (Step-by-Step)

Iodide (I⁻) is actively absorbed from the blood into follicular cells.

Iodide is oxidized into iodine (I) by the enzyme thyroid peroxidase.

Iodine binds to tyrosine residues on thyroglobulin forming:

MIT (Mono-iodotyrosine)

DIT (Di-iodotyrosine)

Coupling reaction produces:

DIT + DIT = T4

MIT + DIT = T3

These hormones are stored in the colloid and released into the bloodstream when needed.

Note: Iodine is essential for thyroid hormone formation. Without iodine, T3 & T4 cannot be synthesized.

🧠 Regulation of Thyroid Function

Controlled by the Hypothalamus–Pituitary–Thyroid Axis:

Hypothalamus releases → TRH (Thyrotropin Releasing Hormone)

TRH stimulates pituitary to release → TSH

TSH stimulates thyroid gland to produce → T3 & T4

Negative Feedback Loop

High T3/T4 levels → inhibit TSH & TRH production

Low T3/T4 levels → increase TSH & TRH release
→ This keeps hormone levels balanced.

🌿 Major Functions of Thyroid Hormones

Regulation of Metabolism

Increases BMR (Basal Metabolic Rate)

Stimulates breakdown of carbohydrates, fats, and proteins

Growth & Development

Essential for normal brain development and physical growth in children

Body Temperature Control

Helps maintain thermoregulation

Heart & Circulatory System

Increases heart rate and cardiac output

Cholesterol Regulation

Reduces blood cholesterol levels

Nervous System

Improves alertness, reflexes, mood, and memory

Calcium Homeostasis (via Calcitonin)

Prevents excess calcium in blood by promoting deposition in bones

⚠ Thyroid Disorders
1. Hypothyroidism (Low hormone levels)

Symptoms:

Weight gain

Fatigue and muscle weakness

Dry skin, hair loss

Cold intolerance

Slow heart rate

Depression, poor memory

Types:

Cretinism → occurs in children (growth and mental retardation)

Myxedema → occurs in adults (severe hypothyroidism)

2. Hyperthyroidism (Excess hormone levels)

Symptoms:

Weight loss

Heat intolerance, sweating

Rapid heartbeat (tachycardia)

Anxiety, irritability

Tremors

Bulging eyes (Exophthalmos)

Disease example:

Graves’ disease → autoimmune disorder causing excess T3/T4

3. Goiter (Gland enlargement)

Cause:

Mainly due to iodine deficiency

Neck swelling, difficulty in breathing or swallowing if severe

4. Thyroid Nodules & Cancer

Symptoms:

Neck lump

Change or hoarseness in voice

Difficulty swallowing or breathing due to compression

Some nodules may secrete excess hormones

💊 Treatment Overview

Hypothyroidism → hormone replacement (Levothyroxine)

Hyperthyroidism → antithyroid drugs, radioactive iodine therapy, or surgery

Goiter → iodine supplementation in deficiency cases
(All treatments must be under medical supervision)