Thyroid Gland

Thyroid Hormones

The thyroid gland produces:

• Thyroxine (T4) and triiodothyronine (T3) – hormones that regulate metabolic rate
• Calcitonin – involved in calcium metabolism

T3 and T4 increase the metabolic rate of the body by increasing nuclear transcription of a large number of genes. Production and release of thyroid hormones are stimulated by thyroid-stimulating hormone (TSH), produced by the anterior pituitary gland, and regulated by the hypothalamic–pituitary–thyroid (HPT) axis. Via a negative feedback loop mechanism, high levels of T4 and T3 inhibit the release TRH of thus TSH, to prevent overproduction of thyroid hormones.

Adobe stock

Fig 1: Diagram of thyroid hormone regulation showing hypothalamus, pituitary gland, TRH, TSH, thyroid gland and negative feedback with T3 and T4.

T4 accounts for approximately 93% of hormone secreted by the thyroid gland, with T3 comprising the remaining 7%. T4 is peripherally converted to T3, meaning both hormones are physiologically important. T3 is approximately four times more potent than T4 but has a shorter half-life and is therefore present in lower circulating concentrations.

For normal thyroxine synthesis, approximately 150 micrograms of iodine per day (≈1 mg/week) is required. To prevent iodine deficiency, iodised table salt is widely used. Dietary sources of iodine include fish, eggs and dairy products.

Hypothyroidism

Hypothyroidism is caused by any defect that decreases the production of thyroid hormone to below adequate levels, drugs to treat hypothyroidism include liothyronine and levothyroxine, further reading is available: Drugs For Hypothyroidism. Symptoms of hypothyroidism include bradycardia, weight gain, hair loss and depresssion.

Hyperthyroidism

Hyperthyroidism refers specifically to hyperfunction of the thyroid gland and there are several treatment options available including antithyroid drugs, radioiodine therapy, and surgery. Further reading is available on available pharmacotherapies: Drugs For Hyperthyroidism. Symptoms of hyperthyroidism are often the opposite of those seen in hypothyroidism, including tachycardia, thinning hair and weight loss.

By TeachMeSeries Ltd (2026)

Fig 1: Hypothyroidism and hyperthyroidism symptoms