What the TSH test shows
TSH — thyroid-stimulating hormone, or thyrotropin — is not made by the thyroid. The pituitary gland at the base of the brain releases it to tell the thyroid how much hormone to make, so it is the first-line check of thyroid function, as MedlinePlus explains.
Crucially, TSH moves opposite to thyroid activity. The pituitary tracks thyroid hormone — mainly free T4, which tissues convert into the more active free T3 — and puts out more TSH when it runs low, less when high. The loop is so sensitive that a small hormone change swings TSH widely, so it shifts before free T4 and makes the best screening test, as StatPearls describes.
So a high TSH usually means an under-active thyroid and a low TSH an over-active one — the reverse of what many expect. TSH shows that the thyroid is off, not why, and it assumes a healthy pituitary, so free T4 is read alongside it.
TSH normal range
TSH is reported in mIU/L, numerically identical to µIU/mL, so US and international reports use the same numbers. Typical adult orientation values:
| Group | Orientation, mIU/L (= µIU/mL) |
|---|---|
| Adults (general) | ~0.4–4.0 |
| Adults over ~70–80 | upper limit drifts higher, up to ~5–6 |
| Pregnancy, 1st trimester | ~0.1–4.0 (trimester-specific) |
| Pregnancy, 2nd–3rd trimester | ~0.2–4.0 (trimester-specific) |
| Children / newborns | age-specific and higher — use the lab’s range |
A few things move the number without any disease. TSH runs on a daily rhythm (highest overnight, lowest mid-afternoon) and rises with age, so a level near 5 can be normal at 80; pregnancy also lowers it, especially early. Ranges depend on the lab, sex, age and (in pregnancy) trimester — read your result against your own report.
Why TSH is high
A high TSH points to hypothyroidism — the thyroid can’t keep up, so the pituitary raises the signal. It is the commonest thyroid abnormality, more so in women and with age. By frequency:
- Autoimmune (Hashimoto’s) thyroiditis — the leading cause in iodine-sufficient countries like the US, driven by TPO antibodies that damage the gland, per StatPearls. Globally, iodine deficiency leads.
- Subclinical hypothyroidism — a mildly raised TSH (often 4–10) with normal free T4; very common, often with few symptoms.
- After thyroid treatment — surgery, radioactive iodine or neck radiation, or too low a levothyroxine dose.
- Medications — lithium, amiodarone, interferon and some cancer immunotherapy drugs.
- Rare — recovery from transient thyroiditis, or a TSH-producing pituitary tumour (high TSH with high free T4).
When is it urgent? A markedly high TSH with severe symptoms — deep fatigue, cold intolerance, slow pulse or confusion — signals severe hypothyroidism; its extreme form, myxedema coma, is an emergency. High TSH in a newborn or pregnancy needs prompt treatment because it affects brain development, as the American Thyroid Association notes.
Why TSH is low
A low TSH usually means hyperthyroidism — too much thyroid hormone switching the signal off. Less common than the high side, it includes some acutely dangerous states:
- Graves’ disease — the leading cause; autoimmune TSH-receptor antibodies drive the thyroid to overproduce.
- Toxic nodules — a single overactive nodule or a toxic multinodular goiter, more common with age.
- Thyroiditis — subacute, postpartum or silent thyroiditis leaks stored hormone and suppresses TSH briefly.
- Too much thyroid medication — over-replacement with levothyroxine is a very common cause.
- Excess iodine or early pregnancy — amiodarone, CT contrast and kelp raise iodine; first-trimester hCG mildly lowers TSH normally.
One key exception: in central hypothyroidism (a failing pituitary or hypothalamus) TSH can be low with a low free T4 — here it means an under-active thyroid, not an overactive one.
When is it urgent? A very low or undetectable TSH with a high free T4 and symptoms — fast or irregular pulse (especially new atrial fibrillation), weight loss or tremor — is overt hyperthyroidism and needs prompt care. Thyroid storm (high fever, racing heart, confusion) is life-threatening.
What to test alongside
TSH is a screen, not a diagnosis; an abnormal value is followed up with:
- Free T4 — the essential partner, added automatically by many labs; it grades overt vs subclinical disease.
- Free T3 — mainly when hyperthyroidism is suspected.
- TPO antibodies — confirm Hashimoto’s when TSH is high.
- TSH-receptor antibodies — confirm Graves’ disease when TSH is low.
- Thyroglobulin antibodies — a second autoimmune marker.
- Ferritin and vitamin D — deficiencies that mimic the fatigue of an under-active thyroid.
- Total cholesterol and LDL cholesterol — often raised in hypothyroidism, improving once treated.
- Hemoglobin — hypothyroidism is a recognised cause of mild anemia.
What to do about an abnormal result
- Don’t self-treat. Never start, stop or change a thyroid-hormone dose yourself, and don’t take iodine or “thyroid support” supplements to fix a number.
- Watch for biotin. High-dose biotin (vitamin B7) can distort thyroid assays and mimic disease; stop it about two days before the draw.
- Repeat before you worry. A single mildly abnormal TSH is often rechecked after 6–8 weeks with free T4; the USPSTF advises confirming an abnormal screen on a repeat sample.
- High TSH: the doctor checks free T4 and TPO antibodies; overt hypothyroidism is treated with levothyroxine, the dose guided by repeat TSH.
- Low TSH: free T4, free T3 and TSH-receptor antibodies, sometimes an ultrasound or uptake scan, sort out the cause.
- Who to see. Start with your primary-care doctor or GP; they refer to an endocrinologist for overt disease, nodules, pregnancy, or unclear results.
Mini-FAQ
Does a high TSH mean my thyroid is overactive or underactive?
A high TSH almost always means the thyroid is under-active (hypothyroidism). TSH is the pituitary’s signal, so it rises when the thyroid makes too little hormone and falls when it makes too much — the number moves opposite to thyroid activity.
What is a normal TSH level?
Most adult labs use roughly 0.4–4.0 mIU/L (the same as µIU/mL). The upper limit is higher in older adults and differs in pregnancy, and every lab sets its own range, so read your result against the range on your report.
What is subclinical hypothyroidism?
It means TSH is mildly high — often 4–10 mIU/L — while free T4 is still normal. It is common, often causes few or no symptoms, and is usually rechecked in a few weeks before deciding whether treatment is needed.
Can anything throw off a TSH result?
Yes. Recent illness, time of day, pregnancy and some medicines shift TSH, and high-dose biotin supplements can distort the assay — stop biotin about two days before the test. A single borderline value is usually repeated before any diagnosis.
Which other tests are done with TSH?
Free T4 is the usual partner and many labs add it automatically when TSH is abnormal. Free T3 and thyroid antibodies (TPO or TSH-receptor) help find the cause, and ferritin or vitamin D are often checked because they cause similar fatigue.
