Thyroid Function Blood Test

Usually, the thyroid gland in the neck and the pituitary gland in the brain work in harmony to effectively regulate your metabolism and keep you feeling well.

The brain produces Thyroid Stimulating Hormone (TSH) to prompt the thyroid to produce two hormones, thyroxine (T4) and triiodothyronine (T3).

As the levels of these two hormones rise, production of TSH in the brain is down regulated. As the levels of T3 and T4 fall, the brain produces more TSH to stimulate the thyroid gland to work harder.

Testing these three hormones together tells us how well your thyroid is functioning, helps to check pituitary function, and may even help us detect a problem with your thyroid before you have symptoms.

T3 and T4 exist in the blood in ‘free’ form or bound to proteins. The free forms are the active forms and these are what we measure to give an accurate impression of your thyroid function. Most tests for thyroid function just check TSH and T4, but a minority of people are unable to convert T4 to the more active T3 and so we check all 3 hormones to ensure that all elements of your thyroid function are working properly.

When the thyroid is underactive, everything essentially slows down. This results in weight gain, feeling cold all the time, dry skin and hair loss, and feeling tired and low. If the thyroid is overactive, everything speeds up. This causes weight loss, a racing pulse, tremor, feeling jittery and anxious, and the feeling of being too hot all the time.

Even if your blood tests are normal, you should see a doctor if you have any of the symptoms below because they may be a sign of serious underlying illness. 

• Unexplained weight loss.
• Drenching sweats during the night.
• Persistent fevers.
• Progressively severe symptoms of any kind.

This test should be taken after any symptoms of short term illness have resolved.

If you are taking biotin supplements, stop taking them 72 hours before the test.

How do you test thyroid Function?

To test thyroid function we test for the two hormones that are produced by the thyroid (thyroxine and triiodothyronine). These are the active hormones that control your metabolism. Low levels suggest the thyroid is underactive, high levels suggest it is overactive. We also test the hormone secreted from the brain called Thyroid Stimulating Hormone (TSH). This helps us to establish whether thyroid problems are due to a problem with the thyroid or, more rarely, with the brain.

Why do you need to check for three different hormones?

Looking at the levels of T4, T3 and TSH together gives us a lot of information about the underlying cause of any thyroid disorder and helps us to establish where exactly the problem lies. Checking these three together can also help us identify early disease that may not yet be clinically apparent.

Should I test for antibodies as well?

Most thyroid problems are caused by autoimmune diseases – most commonly Grave’s disease and Hashimoto’s thyroiditis. The presence of autoantibodies helps us to establish that this is the case. Some people will have these antibodies but will not have a problem with thyroid function. These people may never develop thyroid disease, but they are at higher risk of doing so and may choose to have regular monitoring so that any problem is picked up early.

If you would like to test for antibodies then please have a look at our Ultimate Thyroid Function Blood Test.

• To see if any symptoms you have such as fatigue, weight change, or mood disturbance are related to a thyroid disorder.
• If you have a family history of thyroid problems and are concerned you may have developed a thyroid disorder.
• To monitor your thyroid function over time.

The hormones secreted by the thyroid gland regulate your metabolism. Because the thyroid is so important, when it goes wrong it can cause wide ranging and disabling symptoms.

The Thyroid Function Blood Test is a good option for those suffering with generalised symptoms such as malaise, fatigue, changes in weight or mood that may be due to thyroid disease.