What does the thyroid do?
The thyroid is a butterfly-shaped gland in your neck that produces hormones — primarily thyroxine (T4) and triiodothyronine (T3). These hormones regulate nearly every cell in the body: your metabolic rate, heart rate, body temperature, weight, mood, digestion, bone health and reproduction. When the thyroid makes too little or too much hormone, virtually every body system is affected.
Thyroid blood test normal ranges
| Test | Normal Range | Interpretation |
|---|---|---|
| TSH | 0.4–4.0 mIU/L | The primary screening test — HIGH TSH = underactive |
| Free T4 (FT4) | 0.8–1.8 ng/dL | LOW T4 + HIGH TSH = hypothyroidism |
| Free T3 (FT3) | 2.3–4.1 pg/mL | Low in hypothyroidism, high in hyperthyroidism |
| Anti-TPO antibodies | <35 IU/mL | High = Hashimoto's thyroiditis (autoimmune) |
| Anti-TSH receptor Ab | Negative | Positive = Graves' disease (autoimmune hyperthyroidism) |
Hypothyroidism — underactive thyroid
UNDERACTIVE High TSH + Low T4
Hypothyroidism means the thyroid isn't making enough hormone. Your pituitary gland raises TSH to try to stimulate the thyroid — so a high TSH is the earliest sign. The most common cause worldwide is Hashimoto's thyroiditis (autoimmune). Other causes include iodine deficiency, thyroid surgery, and radiation treatment. Subclinical hypothyroidism is when TSH is high but T4 is still normal — the thyroid is struggling but compensating.
Symptoms of hypothyroidism
- Unexplained weight gain despite no change in diet
- Persistent fatigue and sluggishness
- Feeling cold all the time
- Hair thinning or hair loss
- Dry skin and brittle nails
- Constipation
- Depression and slow thinking ("brain fog")
- Heavy or irregular menstrual periods
- Elevated cholesterol (thyroid regulates lipid metabolism)
Hyperthyroidism — overactive thyroid
OVERACTIVE Low TSH + High T4/T3
Hyperthyroidism means the thyroid is producing too much hormone. TSH is suppressed (very low) because the pituitary is trying to slow the thyroid down. The most common cause is Graves' disease (an autoimmune condition where antibodies stimulate the thyroid). Other causes include toxic multinodular goitre, thyroid nodules, and thyroiditis (temporary inflammation releasing stored hormone).
Symptoms of hyperthyroidism
- Unintentional weight loss despite increased appetite
- Rapid or irregular heartbeat (palpitations)
- Tremor (shaking hands)
- Feeling hot and sweating excessively
- Anxiety, nervousness, irritability
- Diarrhoea or frequent bowel movements
- Difficulty sleeping
- Bulging eyes (exophthalmos) — specific to Graves' disease
- Muscle weakness
Thyroid nodules and goitre
A goitre is an enlarged thyroid gland — visible as a swelling at the base of the neck. It can occur with hypothyroidism, hyperthyroidism or even a normally functioning thyroid. Thyroid nodules (lumps in the thyroid) are very common — about 50% of people over 50 have them. Most are benign. Large nodules or those with suspicious features on ultrasound may need a fine needle aspiration biopsy (FNAC) to rule out thyroid cancer, which is relatively rare but highly treatable when caught early.
Treatment overview
| Condition | Common Treatment |
|---|---|
| Hypothyroidism | Daily levothyroxine (synthetic T4) tablet — usually lifelong |
| Subclinical hypothyroidism | Monitor or treat if TSH >10 or symptoms present |
| Hyperthyroidism (Graves') | Antithyroid drugs (methimazole/carbimazole), radioactive iodine, or surgery |
| Hashimoto's | Levothyroxine if hypothyroid; monitor if euthyroid |
Questions to ask your doctor
- Should I check my Anti-TPO antibodies?
- What TSH level should I aim for on treatment?
- Do I need a thyroid ultrasound?
- How long before I feel better on levothyroxine?
- Does my diet (iodine, soy, gluten) affect my thyroid?