Total Thyroxine (T4) Blood Test


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.

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Total Thyroxine (T4) Blood Test

About this test

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 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. 

How it works

Collect Sample

We send you an easy-to-use kit to collect your blood sample.

Post Sample

Post your sample to our lab in the prepaid envelope provided.

View Results

View results securely in your own personal dashboard.

Important Information

Our tests are not a substitute for seeing your doctor, especially if you are suffering symptoms. Our doctors will interpret your results based on the information you have provided, but will not diagnose, consult or provide any treatment. You will be advised to see your doctor for any necessary follow-up action.

Tests Included

Thyroid Function

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. 

Finger-prick blood sample collection kit

We will send you your Total Thyroxine (T4) Blood Test finger-prick blood sample collection kit which contains everything you need to take your blood 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 option during the checkout process.

How to collect a finger-prick blood sample

Postage & packaging

Your Total Thyroxine (T4) Blood Test includes 1st class postage and packaging for you to send your blood 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 sample guaranteed next day delivery for extra reassurance.

Private laboratory analysis

Your blood 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.

Interpretation of results

Our medical team will comment on out-of-range blood results and give you follow-up advice where necessary. If you need it, a PDF copy of your Total Thyroxine (T4) Blood Test results can be downloaded for your doctor.

Secure online account

Once you have placed your order you can visit where you can manage your account, track your orders and view your Total Thyroxine (T4) Blood Test results.

Health and Lifestyle tracker

Stay motivated by filling in your online health and lifestyle questionnaire and seeing how improvements in your lifestyle can influence your results. Your medical and family history gives us vital information when interpreting your results.

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