High TSH
TSH is the pituitary gland’s message to your thyroid. When the thyroid slows down, the pituitary shouts louder, so TSH goes up. A high TSH is the earliest sign of an underactive thyroid, but mild rises often settle by themselves.
Quick answer: TSH above about 4.0 mIU/L is high. With a low free T4, it means an underactive thyroid. With a normal free T4 it’s subclinical, and the test is usually repeated in 3 months because many settle. Treatment is generally recommended when TSH is 10 or more on two tests, and sooner in pregnancy.
What the test measures
Thyroid stimulating hormone (TSH) is made by the pituitary gland in the brain and tells the thyroid to make thyroxine (T4). Because TSH reacts strongly to small changes in thyroid hormone, it’s the best first test. If it’s abnormal, labs usually add free T4.
Typical adult ranges
- TSH: about 0.4–4.0 mIU/L (some labs go up to 4.5–5.0). mIU/L and µIU/mL are the same number.
- TSH tends to run a little higher in older people and varies through the day, peaking overnight.
- Pregnancy uses its own, lower trimester ranges.
Two patterns:
- High TSH, low free T4: overt underactive thyroid (hypothyroidism). Usually treated.
- High TSH, normal free T4: subclinical hypothyroidism. Often watched first.
What a high result usually means
Common causes:
- Hashimoto’s thyroiditis, an autoimmune condition and the commonest cause in Australia. Thyroid antibodies (TPO antibodies) are usually positive.
- Recovery from an illness: TSH can rise for a while as you get better. Retest when you’re well.
- After thyroiditis, including after having a baby, or after treatment for an overactive thyroid.
- Not enough levothyroxine, or taking it with food, coffee, iron or calcium, which block absorption.
- Medicines: amiodarone, lithium and some cancer immunotherapies.
Less common but important:
- Iodine deficiency (still common in parts of the world).
- Rare pituitary causes and thyroid hormone resistance.
- Lab interference: high-dose biotin supplements and unusual antibodies can give false results.
Symptoms
Tiredness, feeling cold, weight gain, constipation, dry skin, thinning hair, low mood, heavy periods and a slow pulse. Many people with a mildly high TSH have no symptoms at all. See underactive thyroid.
What happens next
- Repeat TSH and free T4 in about 3 months (NICE), when you’re well, before deciding on treatment.
- TPO antibodies: if positive, the thyroid is more likely to slow down further over time.
- Treatment with levothyroxine is generally considered when TSH is 10 mIU/L or more on two tests 3 months apart. Under 65 with symptoms and TSH above range but under 10, a 6-month trial may be offered.
- If untreated, recheck yearly if you have thyroid antibodies or past thyroid problems, or every 2–3 years if not.
- Pregnant or trying? Tell your GP promptly: targets are lower and treatment starts at lower levels.
- Stop biotin (often in hair, skin and nail supplements) for a few days before testing; ask your lab how long.
See your GP within a few days if you’re pregnant or planning pregnancy and your TSH is high, or if you have a new neck lump or hoarse voice. Get urgent help (000 in Australia, or your local emergency number) for someone with known or likely underactive thyroid who becomes very drowsy or confused, unusually cold or has a very slow pulse: severe hypothyroidism (myxoedema coma) is rare but serious.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Common questions
What TSH level is too high?
Anything above your lab’s range, usually about 4.0–5.0 mIU/L, is high. Treatment is generally recommended at 10 mIU/L or more on two tests 3 months apart, or with a low free T4. Between the top of the range and 10, the decision depends on symptoms, age, antibodies and pregnancy plans.
Is a TSH of 6 bad?
It’s mildly raised. If your free T4 is normal, it’s subclinical hypothyroidism, and the usual step is to repeat the test in about 3 months, because mild rises often return to normal. A thyroid antibody test helps predict whether it will progress.
Can illness or stress raise TSH?
Recovering from an illness can push TSH up for a few weeks, so tests are best done when you’re well. TSH also varies through the day. There’s no good evidence that everyday stress causes a meaningful rise.
Does biotin affect thyroid tests?
Yes. High-dose biotin, common in hair, skin and nail supplements, can make thyroid results falsely high or low. Stop it for a few days before your blood test and tell your doctor you take it.
Do I need levothyroxine for a high TSH?
Not always. It’s usually recommended if TSH is 10 or more on two occasions, if free T4 is low, or in pregnancy. For milder rises, many people are simply monitored, though a trial may help younger people with symptoms.
What about a high TSH in pregnancy?
Tell your GP or midwife promptly. Pregnancy has lower TSH targets, and an underactive thyroid matters more because the baby relies on your thyroid hormone early on. Treatment, if needed, is safe in pregnancy.
Evidence and more information
- NICE NG145: Thyroid disease, assessment and management (subclinical hypothyroidism thresholds and monitoring)
- Pathology Tests Explained (RCPA): Thyroid stimulating hormone (TSH)
- Pathology Tests Explained (RCPA): Thyroid antibodies
- Pearce et al, European Thyroid Journal 2013: ETA guideline on management of subclinical hypothyroidism
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