Thyroid blood tests: TSH and T4 explained

What do TSH and T4 show?

The thyroid is a gland in the neck that makes thyroid hormones. TSH is linked to the pituitary gland, and a pituitary problem can cause too much or too little of it. A medical review describes TSH as the main test for finding problems with the thyroid gland itself. Free T4 is measured to tell overt from subclinical hypothyroidism when the TSH is raised. Results are read as a pattern of TSH and free T4 together, not one number alone.

What pattern suggests an underactive thyroid?

A high TSH with a low free T4 suggests an underactive thyroid, called hypothyroidism. The NHS says an underactive thyroid is diagnosed with blood tests for TSH and T4. The NHS says diagnosis of an underactive thyroid also usually involves a neck examination. Thyroid UK says some people continue to have symptoms while taking levothyroxine.

What pattern suggests an overactive thyroid?

A British Thyroid Foundation leaflet says an overactive thyroid, or hyperthyroidism, is diagnosed with a blood test measuring thyroid hormones.

What if the results are borderline?

A slightly raised TSH with a normal free T4 is called subclinical hypothyroidism. The British Thyroid Foundation says blood TSH levels return to normal in around half of people with subclinical thyroid disease. A test for thyroid antibodies helps to judge that risk. The British Thyroid Foundation says there is little clinical evidence that treatment gives long-term benefit in subclinical thyroid disease. MedlinePlus says abnormal T4 levels can sometimes be a sign of a pituitary problem that causes too much or too little TSH.

Can illness or medicines change the result?

Abnormal T4 levels do not always mean a condition that needs treatment. MedlinePlus says pregnancy, oestrogen levels, liver problems and severe body-wide illness can affect T4 results.

What happens after a diagnosis?

Underactive thyroid is usually treated with daily hormone replacement tablets called levothyroxine. The NHS says this treatment is long term to prevent symptoms. NHS inform says a mild case with no symptoms may not need treatment. Regular blood tests check the dose is right, and NHS inform says side effects usually only occur with too much levothyroxine. Thyroid UK says that if symptoms do not improve on levothyroxine, TSH is measured again and the dose may be adjusted. Pregnancy often means the levothyroxine dose needs to be changed, according to the NHS.

What can cause an underactive or overactive thyroid?

In the UK the main cause of an underactive thyroid is an autoimmune condition called Hashimoto's disease. Symptoms of an underactive thyroid tend to appear gradually and can be mild, such as tiredness, weight gain, constipation and low mood. Graves' disease is the most common cause of an overactive thyroid. Symptoms of an overactive thyroid include nervousness, poor sleep, a fast or irregular heartbeat, trembling and weight loss. Treatment for an overactive thyroid can include anti-thyroid medicines, beta-blockers for symptoms such as tremor, and radioactive iodine. Radioactive iodine often leads to an underactive thyroid afterwards. TSH receptor antibodies are associated with Graves' disease, so antibody tests can help find the cause of an overactive thyroid. Some people with Graves' disease develop eye problems, known as thyroid eye disease. UCLH says radioactive iodine may worsen thyroid eye disease, so treatment may be postponed until the eye problems are stable or given with steroid or thyroid medication cover. Beta-blockers can relieve symptoms of an overactive thyroid such as tremor and a fast heartbeat, but they do not affect how much thyroid hormone is made.