What is TSH?
TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland in the brain. It tells the thyroid gland how much thyroid hormone (T3 and T4) to produce. TSH works like a thermostat: if thyroid hormone levels are low, the pituitary pumps out more TSH to stimulate the thyroid. If thyroid hormone is too high, TSH drops to signal the thyroid to slow down. This is why TSH moves in the opposite direction to thyroid hormones.
TSH Normal Range
| Group | Normal TSH (mIU/L) |
|---|---|
| Adults (general) | 0.4 – 4.0 mIU/L |
| Adults (many labs) | 0.5 – 5.0 mIU/L |
| Pregnancy (1st trimester) | 0.1 – 2.5 mIU/L |
| Pregnancy (2nd trimester) | 0.2 – 3.0 mIU/L |
| Pregnancy (3rd trimester) | 0.3 – 3.0 mIU/L |
| Elderly (>70 yrs) | Up to 6.0 may be acceptable |
What does HIGH TSH mean?
HIGH TSH above 4.5 mIU/L — Hypothyroidism
High TSH means your thyroid is underperforming — the pituitary is shouting louder to try to get the sluggish thyroid to produce more hormone. This is hypothyroidism (underactive thyroid). Most common cause: Hashimoto's thyroiditis (autoimmune attack on the thyroid). Symptoms: fatigue, weight gain, feeling cold, constipation, hair loss, dry skin, depression, heavy periods, brain fog. Treatment: levothyroxine tablet once daily, usually lifelong. Most people feel dramatically better within 4–8 weeks of starting treatment.
What does LOW TSH mean?
LOW TSH below 0.4 mIU/L — Hyperthyroidism
Low TSH means your thyroid is overproducing hormones — the pituitary has gone quiet because there's already too much thyroid hormone. This is hyperthyroidism (overactive thyroid). Most common cause: Graves' disease (autoimmune), toxic nodular goitre, or excess thyroid medication. Symptoms: weight loss despite increased appetite, palpitations (fast/irregular heartbeat), anxiety, trembling hands, heat intolerance, sweating, diarrhoea, bulging eyes (Graves'). Requires prompt treatment — untreated hyperthyroidism can cause atrial fibrillation and bone loss.
Subclinical thyroid disease
Subclinical Hypothyroidism: high TSH + normal T4
TSH between 5–10 mIU/L with normal free T4 = subclinical hypothyroidism. Many people have no symptoms. Whether to treat depends on TSH level, symptoms, age, and pregnancy status. Pregnant women with subclinical hypothyroidism are always treated — it affects fetal brain development.
Questions to ask your doctor
- Should I also check free T3 and free T4?
- Should I test thyroid antibodies (anti-TPO) to check for Hashimoto's?
- My TSH is 5.5 — do I need medication or just monitoring?
- I'm pregnant — what TSH level is safe for my baby?