TSH test: normal range, full form and what high or low TSH means

TSH stands for thyroid-stimulating hormone. It isn't made by the thyroid at all: it comes from the pituitary gland at the base of the brain, and its job is to tell the thyroid in your neck how much thyroid hormone (T4 and T3) to make.

It works like a thermostat. When thyroid hormone runs low, the pituitary turns TSH up to push the thyroid harder. When there's too much, it turns TSH down. That's why the result can feel backwards: a high TSH usually means an underactive thyroid, and a low TSH usually means an overactive one.

Because TSH reacts to small changes before T4 does, it's the first and most sensitive thyroid test. A thyroid profile usually adds free T4 and sometimes free T3, and thyroid antibodies (anti-TPO) show whether the immune system is attacking the thyroid.

A high TSH most often means the thyroid isn't making enough hormone: hypothyroidism. The most common cause worldwide is Hashimoto's thyroiditis, where the immune system slowly damages the thyroid; others are iodine deficiency, past thyroid surgery or radioactive iodine, and some medicines (such as lithium and amiodarone).

Common signs are tiredness, feeling cold, weight gain, constipation, dry skin, thinning hair, low mood, slow thinking, heavy periods and a slow pulse. Many people have no symptoms at all, especially when TSH is only mildly high. Our thyroid symptoms guide sets both sides out with a checklist.

A mildly high TSH (about 4 to 10) with normal free T4 is called subclinical hypothyroidism. It's common, can come back to normal on its own, and is usually rechecked in 6 to 12 weeks with antibodies before deciding anything. Above 10, or with symptoms, pregnancy or plans for pregnancy, treatment is usually considered.

TSH can also be raised for a while after an illness, and it runs higher in older adults.

A low TSH usually means the thyroid is making too much hormone: hyperthyroidism. Common causes are Graves' disease (an immune condition), thyroid nodules that work on their own, and inflammation of the thyroid (thyroiditis), which is often temporary.

If you take thyroid medicine, a low TSH most often means the dose is higher than you need. Don't change it yourself; tell the doctor who prescribes it.

Signs of an overactive thyroid include weight loss without trying, a fast or pounding heartbeat, anxiety, a shaky hand, feeling hot and sweaty, loose stools, poor sleep and lighter periods. An overactive thyroid can affect the heart rhythm and bones, so a clearly low TSH needs a prompt check.

Less often, TSH is low because of a serious illness, steroid medicines, early pregnancy (when it naturally dips) or, rarely, a pituitary problem.

Pregnancy changes thyroid tests. TSH naturally falls in the first trimester, and the baby relies on the mother's thyroid hormone early on, so labs use trimester-specific ranges. When those aren't available, the American Thyroid Association suggests treating about 4.0 mIU/L as the upper limit in early pregnancy. If you're pregnant or planning to be and have a thyroid condition or an abnormal TSH, see your doctor promptly; thyroid medicine often needs adjusting early.

Older adults naturally run a higher TSH. A value of 5 to 7 can be normal over the age of 70, and doctors are more cautious about treating a mildly high TSH in older people.

• It's a small blood sample from a vein. Fasting isn't needed for TSH, though other tests at the same visit may need it.
• Stop biotin (vitamin B7, found in many hair, skin and nail supplements) for at least 2 days before, as it can make thyroid results look falsely high or low. Ask your doctor first if it was prescribed.
• If you take thyroid medicine, have the blood taken before your morning dose, and test at about the same time of day each time.
• TSH is a little higher in the early morning than in the afternoon, and can shift during or just after an illness.
• Results usually come back the same day or the next.

• Starting or changing thyroid medicine: about 6 to 8 weeks later, because TSH takes that long to settle.
• On a steady dose: usually once a year.
• Mildly high TSH with no treatment: recheck in 6 to 12 weeks, then every 6 to 12 months.
• Pregnancy with a thyroid condition: every 4 weeks or so in the first half of pregnancy, as your doctor advises.
• Risk factors (type 1 diabetes, a family history of thyroid disease, other autoimmune conditions, past neck radiation, or symptoms): ask your doctor about checking.

• Thyroid medicine and its dose are your doctor's decision. Never start, stop or change it on your own; small changes make a big difference.
• Take thyroid medicine the way your doctor or pharmacist explained, usually on an empty stomach and apart from calcium, iron and soy, which block its absorption.
• Use iodised salt in normal amounts; avoid high-dose iodine or kelp supplements unless your doctor recommends them.
• Look after the rest: sleep, activity and a balanced diet help energy and weight whatever your thyroid result.
• Keep a record of your TSH and free T4 over time. The trend often says more than a single value.