Thyroid blood tests
Measuring thyroid hormone levels
Increasingly, patients wish to know about the actual levels of thyroid hormones and TSH in the blood.
These ranges will vary slightly from laboratory to laboratory, depending upon the normal population used for the calculations, and upon the type of chemical analysis used for the measurement of the hormones.
The thyroid hormones, triiodothyronine or T3 and thyroxine or T4, are almost exclusively bound to a protein in the bloodstream and, as such, are inactive. Less than one per cent of these hormones are unbound or free and able to control the metabolism of the body.
Measurement of total T3 (TT3) and T4 (TT4) includes both bound and free fractions, whereas measurement of free T4 (fT4) and T3 (fT3) excludes the much larger bound fraction.
In most circumstances, measurement of free and total thyroid hormones provides the same information about whether the thyroid is working normally or in an over- or underactive fashion. Some hospital laboratories offer the measurement of total thyroid hormones and others of free thyroid hormones, but rarely both.
Typical results in hyperthyroidism and hypothyroidism
Hyperthyroid
Generally speaking, the more severe the symptoms of over- or underactivity of the thyroid gland, the more abnormal the results of the thyroid blood tests. In most patients with hyperthyroidism:
• TT4 would be about 190 nmol/l
• TT3 4 nmol/l
• fT4 40 pmol/l
• fT3 12 pmol/l.
Much higher values may be recorded, however, with fT4 in excess of 100 pmol/l. In older patients in whom hyperthyroidism may be no less serious with heart complications, such as an irregular heartbeat caused by atrial fibrillation, the levels of thyroid hormones may be only marginally elevated. In all patients with hyperthyroidism, with very rare exceptions, the TSH level in the blood is so low that it cannot be detected.
Hypothyroid
By the time that patients with hypothyroidism present with typical symptoms, fT4 and TT4 levels are very low and often less than 5 pmol/l and 20 nmol/l respectively, and associated with a raised TSH level in the blood of more than 30 mU/l.
Rarely, hypothyroidism is the result of disease of the pituitary gland and not of the thyroid gland itself, in which case the low fT4 or TT4 is combined with a normal or low level of TSH.
In mild or subclinical hypothyroidism, fT4 and TT4 lie in the lower part of the normal range, for example 11 pmol/l or 65 nmol/l, and are usually associated with a TSH level in the blood of between 5 and 10 mU/l. Levels of T3 are not usually measured in patientswith suspected hypothyroidism.
Judging the correct dose of thyroxine
Your GP or thyroid specialist will usually prescribe a dose of thyroxine that raises the fT4 and TT4 to the upper part of the normal range and reduces the TSH level in the blood to the lower part of the normal range.
Typical results would be a fT4 of 24 pmol/l or TT4 of 140 nmol/l, and a TSH of 0.2 mU/l. In some patients, a sense of well-being is achieved only when fT4 or TT4 is raised, for example 30 pmol/l or 170 nmol/l, and TSH low or undetectable. In this circumstance, it is essential that the T3 level in the blood is unequivocally normal in order to avoid hyperthyroidism.
Failure to take thyroxine regularly is very obvious from blood test results.
Effect of illness on thyroid blood tests
Illness, whether sudden such as pneumonia or a heart attack, or of long duration, such as rheumatoid arthritis or depression, may affect the results of thyroid blood tests and give the impression of hyper- or hypothyroidism. It is possible that, after referral to a specialist and after further investigations, no underlying thyroid disease will be found.
KEY POINTS
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The normal ranges for thyroid blood tests will vary from laboratory to laboratory
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Generally, the more severe the symptoms the more abnormal the results of the thyroid blood test
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Thyroid blood tests should not be interpreted in isolation
