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Thyroxine (T4)

Thyroxine (T4) is the main hormone produced by the thyroid gland. This blood test measures levels of T4 to help in the diagnosis of thyroid disease.

To produce thyroxine, the thyroid gland needs iodine from the food we eat. Thyroxine is called T4 because it contains 4 atoms of iodine. Although it is the most plentiful of the thyroid hormones, it is not biologically active and does not govern metabolism in our cells. That job is undertaken by triiodothyronine, or T3. T4 converts to T3 by the removal of an atom of iodine.

Thyroxine is typically measured in thyroid function tests because a low T4 reading is likely to indicate insufficient levels of T3 (and therefore a slow metabolism) while high levels of T4 tend to lead to excess T3 (resulting in an overactive metabolism).

Who is this blood test for

This test is for individuals who have symptoms of either an underactive or an overactive thyroid and want to assess whether their thyroid is producing too much or too little thyroxine. It can also be used to monitor an already diagnosed thyroid disorder. 

What's included

T4 (Thyroxine) is the main hormone produced by the thyroid gland. T4 is not biologically active, and must be converted to T3 before it can become active in regulating metabolism. Some T4 is bound to proteins, while some T4 is 'free' or unbound. This test measures the level of total T4 in your blood. 

Raised T4 can indicate an overactive thyroid while low T4 may indicate that the thyroid is struggling to produce sufficient thyroid hormones. 

We will send you your Thyroxine (T4) finger-prick blood and test sample collection kit which contains everything you need to take your blood and sample in the comfort of your own home. If you are unsure about completing a finger-prick blood sample collection you will have the opportunity to select a clinic-based venous blood sample collection or choose to go our London laboratory during the checkout process.

Your Thyroxine (T4) test includes 1st class postage and packaging for you to send your blood and sample directly to our laboratory for analysis. If you live in an area where you cannot rely on the post or you simply want to ensure that your sample arrives at the laboratory the following day, you may wish to send your blood and sample guaranteed next day delivery for extra reassurance.

Your blood and sample will be analysed at one of our chosen laboratories. You can be assured of fast, accurate results from one of our accredited independent providers of clinical diagnostic tests.
Once you have placed your order you will receive login details to where you can manage your account, track your orders and view your Thyroxine (T4) test results in your own personal dashboard.
Our medical team will comment on out-of-range blood and test results and give you follow-up advice where necessary. If you need it, a PDF copy of your Thyroxine (T4) test results can be downloaded for your doctor. Want a hard-copy report? You will be given the opportunity to order one during the checkout process.

Thyroxine (T4) is included in the following categories: Thyroid Health