What Does the Thyroid Do?

The thyroid is a small butterfly-shaped gland at the front of the neck, just below the Adam's apple. Despite its size, it has an outsized influence: the hormones it produces — mainly thyroxine (T4) and triiodothyronine (T3) — regulate metabolism in every cell in the body. They control how fast the heart beats, how quickly food is digested, how the brain develops, body temperature, and the rate at which calories are burned.

The Two Main Disorders

Hypothyroidism — Underactive Thyroid

The thyroid does not produce enough hormone. This slows everything down. It affects an estimated 1 in 20 adults, with women up to 10 times more likely to be affected than men. Autoimmune destruction of the gland (Hashimoto's thyroiditis) is the most common cause.

Symptoms:

  • Persistent fatigue and sluggishness
  • Weight gain despite no change in diet
  • Feeling cold even in warm environments
  • Dry skin, brittle nails, hair thinning or loss
  • Constipation
  • Depression, poor memory and brain fog
  • Slowed heart rate
  • Heavy or irregular periods
  • Swelling in the neck (goitre)

Hyperthyroidism — Overactive Thyroid

The thyroid produces too much hormone, speeding everything up. Graves' disease (an autoimmune condition) is the most common cause.

Symptoms:

  • Unexplained weight loss despite increased appetite
  • Rapid or irregular heartbeat (palpitations)
  • Anxiety, irritability, and restlessness
  • Sweating and sensitivity to heat
  • Tremors in hands and fingers
  • Frequent bowel movements
  • Difficulty sleeping
  • Bulging eyes (in Graves' disease)

How Thyroid Function Is Tested

A blood test measuring TSH (thyroid-stimulating hormone) is the primary diagnostic tool. TSH is produced by the pituitary gland and tells the thyroid how hard to work. A high TSH signals the thyroid is underperforming; a low TSH suggests it is overactive. T3 and T4 levels provide further detail. Antibody tests (anti-TPO, anti-Tg) identify autoimmune causes.

The Role of Iodine and Nutrition

Iodine is essential for thyroid hormone production. Deficiency remains a leading cause of goitre and hypothyroidism worldwide, particularly in landlocked regions with low dietary iodine. Iodised salt and seafood are the primary sources. Selenium is also critical for converting T4 to the active T3 form — Brazil nuts, fish, and eggs are good sources.

Some foods called goitrogens (raw cruciferous vegetables — broccoli, cabbage, kale) can mildly interfere with thyroid function in very large quantities, but are not a concern when eaten normally and cooked.

Treatment

Hypothyroidism is typically treated with a daily oral dose of synthetic T4 (levothyroxine), which is highly effective when the dose is correctly calibrated. TSH levels are monitored every 6–12 months to adjust dosing. Hyperthyroidism is treated with anti-thyroid medications, radioiodine therapy, or surgery depending on severity and cause.

Key Takeaway

Thyroid disorders are common, often subtle, and very treatable. If you have unexplained fatigue, weight changes, or mood shifts, ask your doctor for a TSH blood test — it is a simple, inexpensive, and highly informative test.