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Thyroid Blood Tests
(TFT - Thyroid Function Test)


The blood test for thyroid problems is called a Thyroid Function Test. The most usual tests are:

There is a range, which is used for the tests. These vary sometimes which is why you must always ask what the range is, so that you can see where you are in the range.

This hormone comes from the pituitary to stimulate the thyroid gland into making more hormone. TSH rises when the thyroid is struggling.
The approx. reference range for this test is 0.4 to 4.5.

Thyroid hormones bound to proteins. TT4 lowers when the thyroid is struggling.
The approx. reference range for this test is 50 to 160.

Thyroid hormones not bound to proteins. FT4 lowers when the thyroid is struggling.
The approx. reference range for this test is 10 to 24

T4 converts to T3 and is the only thyroid hormone actually used by the body's cells.
The approx. reference range for Free T3 is 4 to 8.3

We at Thyroid UK believe that you need to know your Free T3 level too because this will often show low if you are not converting, and high if you have blocked receptor cells. Even if you are converting, the body needs the extra T3 that a normal thyroid produces. There has been some research to show that people feel better on a mixture of Thyroxine (T4) and Triiodothyronine (T3). Effects of Thyroxine as Compared with Thyroxine plus Triiodothyronine in patients with hypothyroidism – The New England Journal of Medicine Feb.11, 99 Vol. 340. (Click here for this article).

With all of these tests, your results could be anywhere within the range and you would be classed as "normal". If you are at the very edge of the range, either at the bottom or at the top, you could be classed as "borderline". Neither you nor your doctor truly knows what your normal is, if you did not have a blood test done before you became ill. There are also particular reasons why the blood tests remain in the normal range. If you are not converting from T4 to T3 or if your cells are not taking up the T3 normally, your T4 levels and your TSH levels will still show as normal.

The Broda Barnes Foundation tell us "Dr Barnes found that the primary reason for the inaccuracy of the blood tests for thyroid function is that the thyroid hormones are not utilized in the blood, but are utilized intracellularly. Therefore a patient can have enough thyroid hormones circulating in the blood to give a "normal" reading, but if the hormones are not getting into the cells, the patient will be hypothyroid.

Also, there is a problem of low blood volume in hypothyroid patients. This means that any blood test value will appear higher than it actually is because the patient has a lower than normal blood volume. In many patients with "normal" or "high" thyroid blood levels, but many clinical symptoms, the patient's blood levels actually drop lower once the patient is given thyroid hormones simply because their blood volume increases.

NB: Dr Bayliss tells us in his book Thyroid Disease: the facts "Do not take any thyroxine on the day when you go for your check-up and have a blood sample taken. The reason for this is that for about 4 hours after ingesting a tablet of thyroxine, the blood level of thyroxine peaks and will give a spuriously high reading". Other doctors I have asked tell me that it makes no difference. You decide!!

Taken from Medicine International 1993

"The aim of thyroxine replacement therapy is to normalise plasma TSH and to achieve a clinically euthyroid state. To obtain this, FT4 and TT4 have to be maintained at, or just above, the upper reference interval".

NB: Dr. Toft, Consultant Physician, states in the British Thyroid Foundation newsletter Issue No. 23 that normal ranges are: T4 (10 – 25) and TSH (0.15 – 3.5). He also states that "the correct dose is that which restores good health; in most patients this will be associated with a level of T4 in the blood towards the upper part of the normal range or even slightly high and a TSH level in the blood which is in the lower part of the normal range.

NNB: We suggest that people get an actual copy of their test results. For more information on this, please click here


Myxoedema is the medical term for severe hypothyroidism.
Mucopolysaccharides – Gel-Like Substance.

Myx = mucin (gel-like) - Oedema = swelling.

The body makes mucopolysaccharides in normal quantities. When the thyroid is malfunctioning, they are mass-produced. Hence the bloated look that patients sometimes have. These mucopolysaccharides infiltrate into all the connective tissues and cause many problems i.e. joint pains and muscular pains.

Did you know that the thyroid becomes more effective in the summer? So if your symptoms come back a bit in the winter, you may need a slightly higher dosage of your hormone replacement.


Last updated 8/07/2015