Thyroid function tests

Thyroid function tests are blood tests that show how well your thyroid gland is working. The thyroid is a small gland in the neck. TSH (thyroid-stimulating hormone) is made by the pituitary gland in the brain, and it tells the thyroid how much hormone to make. T4 (thyroxine) and T3 (triiodothyronine) are the hormones the thyroid itself makes. TSH is usually tested first, and T4 and sometimes T3 are added. Thyroid antibody tests may follow.

A doctor may ask for these tests if you have symptoms of too little or too much thyroid hormone. An underactive thyroid can cause tiredness, weight gain, feeling cold, dry skin, thinning hair and constipation. It often develops so slowly that symptoms go unnoticed for months or years. An overactive thyroid can cause weight loss despite normal eating, a fast or irregular heartbeat, nervousness, trouble sleeping, shaky hands and sweating. The tests are also done for lumps in the thyroid, to check the dose of thyroid medicine, to follow treatment for an overactive thyroid, and during pregnancy. In newborn babies, a TSH test screens for an underactive thyroid from birth.

No special preparation is usually needed. Fasting is asked for only if other tests are done on the same sample. TSH changes during the day, so it is best to have the test early in the morning.

The blood sample is taken like this:

1. You sit down and roll up your sleeve. A health worker puts a small needle into a vein in your arm. You may feel a brief sting.
2. Blood runs into a small tube. This usually takes under five minutes.
3. The tube is sent to the lab.

You may have a small bruise where the needle went in. Ask when and how you will get the result, and who will explain it. If you start thyroid medicine, you will have regular blood tests to check that the dose is right.

Thyroid function tests carry very little risk. You may have slight pain or bruising where the needle went in, and this usually fades quickly.

The report lists each result next to the lab's own normal range, which depends on age, sex and the method the lab uses. Compare your result only with the range on your own report. A high TSH fits an underactive thyroid, because the pituitary is pushing a thyroid that is making too little hormone. A low TSH fits an overactive thyroid. A doctor reads TSH with T4 and T3: a low TSH with a high T3 or T4 usually points to an overactive thyroid. Rarely, an abnormal TSH comes from a problem with the pituitary gland.

TSH can change without thyroid disease. Some serious illnesses can lower it for a short time, it is often slightly low in the first three months of pregnancy, and it may be higher in people over 80 who have healthy thyroids. These tests show how much hormone you have, not why. If the results are abnormal, doctors may check thyroid antibodies, which look for autoimmune causes such as Graves' disease (a common cause of an overactive thyroid) and Hashimoto's disease (a common cause of an underactive thyroid).