What Does the Thyroid Do?
The thyroid is a butterfly-shaped gland at the base of the neck that produces thyroxine (T4) and triiodothyronine (T3) — hormones that regulate metabolism in every cell of the body. They govern heart rate, body temperature, energy production, mood, weight, and the function of nearly every organ. Too little thyroid hormone (hypothyroidism) slows everything down; too much (hyperthyroidism) speeds everything up.
Hypothyroidism: Underactive Thyroid
Hashimoto's thyroiditis — an autoimmune condition in which the immune system attacks the thyroid — is the most common cause in developed countries. Other causes include prior thyroid surgery, radioiodine treatment, certain medications (lithium, amiodarone), and iodine deficiency (globally, the most common cause).
Symptoms are often subtle and overlap with many other conditions:
- Fatigue and sluggishness
- Unexplained weight gain
- Feeling cold (especially cold hands and feet)
- Constipation
- Dry skin, brittle nails, hair thinning or loss
- Brain fog, depression, poor memory
- Elevated cholesterol
- In women: heavy or irregular periods
Diagnosis is by TSH (thyroid-stimulating hormone) — elevated TSH (typically >4.5 mIU/L) indicates the pituitary is working overtime because the thyroid is underperforming. Free T4 is measured to confirm the degree of deficiency.
Treatment: Levothyroxine (synthetic T4) is the standard of care, taken once daily in the morning on an empty stomach. The goal is to normalise TSH to 0.5–2.5 mIU/L. Dosage is adjusted based on repeat TSH testing after 6–8 weeks.
Hyperthyroidism: Overactive Thyroid
Graves' disease — another autoimmune condition, in which thyroid-stimulating antibodies (TSI) continuously stimulate the thyroid — causes 80% of hyperthyroidism cases. Other causes include toxic multinodular goitre, toxic adenoma, and thyroiditis (temporary inflammation releasing stored hormone).
Symptoms reflect a revved-up metabolism:
- Rapid or irregular heartbeat (palpitations, atrial fibrillation)
- Unintentional weight loss despite increased appetite
- Heat intolerance and excessive sweating
- Anxiety, irritability, tremors
- Diarrhoea or frequent bowel movements
- Muscle weakness
- Graves' ophthalmopathy: eye bulging, dryness, and double vision (specific to Graves')
Diagnosis: Suppressed TSH (<0.1 mIU/L) with elevated free T3 and/or T4 confirms hyperthyroidism. TSI antibodies confirm Graves' disease.
Treatment options include antithyroid medications (methimazole, propylthiouracil — PTU), radioactive iodine ablation (the most commonly chosen definitive treatment in the US), or surgical thyroidectomy. After radioiodine or thyroidectomy, most patients develop hypothyroidism and require lifelong levothyroxine.
When to Test Your Thyroid
Testing (TSH) is recommended if you have unexplained fatigue, weight changes, mood disturbances, palpitations, or hair loss — or if you're a woman over 35, pregnant, or postpartum (postpartum thyroiditis affects 5–10% of women). Thyroid disease runs strongly in families, so a positive family history lowers the threshold for testing.