See your GP if you think you may have overactive thyroid (hyperthyroidism).
A diagnosis of overactive thyroid will be based on your symptoms and the results of a blood test, which assesses how well your thyroid gland is working. This blood test is known as a thyroid function test.
Thyroid function test
Your GP will take a sample of your blood and test it for levels of:
- thyroid-stimulating hormone (TSH)
- thyroxine and triiodothyronine (the thyroid hormones)
TSH is made in the pituitary gland in your brain and controls the production of thyroxine and triiodothyronine.
- When the level of thyroxine and triiodothyronine in your blood are normal, your pituitary gland stops releasing TSH.
- When the level of thyroxine or triiodothyronine drops, the pituitary gland produces more TSH to boost the level.
If you have overactive thyroid, the thyroid function test will show that the levels of TSH in your blood are consistently lower than normal. Low levels of TSH mean your thyroid gland is overactive and is likely to be making too many thyroid hormones. This is the first part of the thyroid function test.
Your GP will then test your blood for levels of thyroxine and triiodothyronine. If you have overactive thyroid, you will have higher than normal levels of both of these hormones, which causes the symptoms of overactive thyroid.
Types of hyperthyroidism
If your thyroid function test confirms a diagnosis of hyperthyroidism, your GP should be able to tell you whether you have overt (fully developed) overactive thyroid or subclinical overactive thyroid (where your symptoms are mild or absent).
If you have overt overactive thyroid, your blood may have low levels of TSH and high levels of thyroxine or triiodothyronine. Your symptoms are likely to be moderate or severe.
If you have subclinical hyperthyroidism, your blood may have low levels of TSH but normal levels of thyroxine and triiodothyronine. You may have no symptoms at all.
It is possible for subclinical overactive thyroid to develop into overt overactive thyroid.
