TSH (Thyroid Stimulating Hormone)

TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland and tells the thyroid how much thyroid hormone (T4 and T3) to make. It works on a feedback loop: when circulating thyroid hormone is low, TSH rises to stimulate more production; when thyroid hormone is high, TSH falls. Because the feedback loop is so sensitive, TSH typically changes before T4/T3 do — making it the single most useful screening test for thyroid dysfunction. Australian pathology labs report TSH in mIU/L (also written μIU/mL or mU/L — they are equivalent).

TSH is the first-line test for hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). It is one of the most commonly ordered blood tests in Australia, often included in routine GP health checks because thyroid dysfunction is so common — affecting roughly 1 in 10 Australian women and 1 in 20 men over their lifetime, with rates rising sharply after menopause. TSH is also used to monitor treatment with levothyroxine (Eutroxsig, Oroxine) and to assess pituitary function.

High TSH (above 4.0 mIU/L) usually means the thyroid is underperforming and the pituitary is working harder to compensate — primary hypothyroidism. The most common cause in Australia is Hashimoto's thyroiditis (autoimmune destruction of the thyroid), confirmed by elevated TPO antibodies. Other causes include iodine deficiency (uncommon in iodised-salt Australia but possible in vegan or low-dairy diets), post-surgical hypothyroidism (after thyroidectomy), post-radioactive iodine treatment, certain medications (lithium, amiodarone, immune checkpoint inhibitors), and recovery from severe illness. Subclinical hypothyroidism — TSH 4.0–10 with normal free T4 — is increasingly common, especially in postmenopausal women, and is often treated when TSH exceeds 10 or when symptoms are significant. Pregnancy applies stricter limits: TSH should typically be <2.5 mIU/L in the first trimester.

Low TSH (below 0.4 mIU/L) usually means too much thyroid hormone is circulating — primary hyperthyroidism. The leading Australian cause is Graves' disease (autoimmune stimulation of the thyroid), followed by toxic multinodular goitre, toxic adenoma, transient subacute thyroiditis (often post-viral), and iatrogenic causes — the most common being over-replacement on levothyroxine for hypothyroidism. Symptoms include weight loss despite normal eating, palpitations and tachycardia, anxiety, heat intolerance, sweating, tremor, frequent bowel motions and difficulty sleeping. Suppressed TSH with normal T4 and T3 (subclinical hyperthyroidism) is common in elderly Australians on long-term levothyroxine; chronic suppression increases the risk of atrial fibrillation and osteoporosis. Rarely, low TSH can reflect pituitary failure (central hypothyroidism), in which case T4 will also be low — a different clinical picture from primary hyperthyroidism.

TSH is recommended as part of routine annual health checks for adults over 50, postmenopausal women, anyone with a family history of thyroid disease, and during pregnancy planning. People on levothyroxine for hypothyroidism should retest 6-8 weeks after any dose change, then every 6-12 months once stable. Pregnant women should test TSH at the first antenatal visit and every trimester thereafter. Athletes, people on TRT, and anyone presenting with fatigue, weight changes, mood symptoms or hair loss should have TSH tested as part of the workup.

When TSH is abnormal, free T4 (FT4) and sometimes free T3 (FT3) are added to confirm and characterise the dysfunction. Anti-TPO and anti-thyroglobulin antibodies confirm Hashimoto's; TSH-receptor antibodies (TRAb) confirm Graves' disease. Reverse T3 is occasionally measured in suspected non-thyroidal illness syndrome.