Thyroid and Parathyroid Glands: Functions and Hormones

The thyroid gland (in the neck) controls metabolic rate using hormones T3 and T4, while the four small parathyroid glands behind it control blood calcium using parathyroid hormone (PTH). Both are essential for energy and bone health.

These glands matter daily: thyroid hormones affect heart rate, temperature and growth, and precise calcium control is needed for nerves, muscles and blood clotting. Even small imbalances cause noticeable illness.

Key Concepts

• T4 (thyroxine) and T3: iodine-containing thyroid hormones; T3 is more active.
• TSH: pituitary hormone stimulating the thyroid.
• Calcitonin: thyroid C-cell hormone that lowers blood calcium (minor role in adults).
• PTH (parathyroid hormone): raises blood calcium.
• Basal metabolic rate (BMR): resting energy use, strongly influenced by thyroid hormones.

How It Works

1. Pituitary TSH stimulates thyroid follicles to take up iodine and make T3/T4.
2. T3/T4 increase metabolic activity in most tissues and support normal growth and brain development.
3. Parathyroid chief cells sense blood Ca2+ via calcium-sensing receptors.
4. If Ca2+ falls, PTH is released: it increases bone resorption, kidney Ca2+ reabsorption, and active vitamin D formation (increasing gut absorption).
5. Rising Ca2+ then suppresses PTH (negative feedback). Calcitonin opposes mildly when Ca2+ is high.

Worked Example

Problem: Blood calcium falls below normal (~2.2–2.6 mmol/L total calcium range in many labs). What happens?

Step 1: Parathyroid calcium sensors detect low Ca2+.

Step 2: PTH secretion increases within minutes.

Step 3: Kidneys retain more Ca2+ and activate vitamin D; gut absorption rises over hours.

Step 4: Bone releases some Ca2+ into blood.

Step 5: Blood Ca2+ returns toward normal and PTH falls again.

Common Misconceptions

• Myth: Goitre always means overactive thyroid. Correction: Goitre is enlargement, often from iodine deficiency with low hormone output, though it can occur in overactivity too.
• Myth: Parathyroid is part of thyroid function. Correction: They are separate glands with opposite calcium actions and different hormones.
• Myth: More thyroid hormone is always better for energy. Correction: Excess causes weight loss, tremor and dangerous heart rhythm problems; balance is required.

Real-World Applications

Iodised salt prevents iodine-deficiency goitre and cretinism. Hypothyroidism is treated with levothyroxine (synthetic T4). During thyroid surgery, preserving parathyroids is critical to avoid lifelong calcium problems.

Thyroid sets the body's metabolic speed; parathyroids guard blood calcium minute by minute — two neighbouring glands with entirely different jobs. Thyroid hormones also influence cholesterol metabolism, gut motility, menstrual regularity and mood, which explains why hypothyroid patients often report constipation, cold intolerance, weight gain and low mood together, while hyperthyroid patients report heat intolerance, diarrhoea, anxiety and palpitations, a pattern doctors use alongside TSH and free T4 blood tests to confirm the diagnosis before starting treatment.