See a GP if you think you may have an overactive thyroid (hyperthyroidism). A diagnosis will be based on your symptoms and the results of a blood test that assesses how well your thyroid is working.

Thyroid function blood test

A GP may arrange for you to have a blood test to check your thyroid hormone levels. This is known as a thyroid function test. The test checks the levels of:

• thyroid-stimulating hormone (TSH) – a hormone produced by the pituitary gland (a gland at the base of the brain) which controls the production of thyroid hormones
• triiodothyronine (T3) – one of the main thyroid hormones
• thyroxine (T4) – another of the main thyroid hormones

Your thyroid hormone levels will be compared to what's normal for a healthy person of your age. A low level of TSH and high levels of T3 and/or T4 usually means you have an overactive thyroid.

Doctors may refer to these measurements as "free" T3 and T4 (FT3 and FT4). What's considered normal varies depending on things such as your age and the exact testing technique used by the laboratory.

Further blood tests

You may have another blood test to look for anti-thyroid antibodies. These are usually found if you have Graves' disease, a common cause of an overactive thyroid.

A blood test called erythrocyte sedimentation rate (ESR) may also be done to check for inflammation in your body. If there are signs of inflammation, the increase in thyroid hormones is caused by thyroiditis (inflammation of the thyroid).

Thyroid scan

A thyroid scan may be used to look for problems such as lumps (nodules) on your thyroid. You'll be asked to swallow a small amount of a slightly radioactive substance that will be absorbed by your thyroid. It can also be given by injection. A scan is then done to see how much of the substance has been absorbed and to examine the size and shape of your thyroid.