The thyroid function test comprises of a panel of blood tests that can be used by doctors to diagnose thyroid problems. Most of the blood tests that might potentially be included in the panel measure the amount of the particular hormone in a given volume of blood (serum); they could include:
- Thyroid Stimulating Hormone (TSH) – produced and released into the bloodstream by the pituitary gland. It controls the production of thyroxine (T4) and triiodothyronine (T3). TSH rises when the blood plasma levels of T3 and T4 fall and vice versa
- Total Thyroxine (Total T4 or TT4) – the total amount of T4, both unbound and bound to proteins, per unit volume of blood plasma
- Free Thyroxine (Free T4) – the amount of unbound T4 per unit volume of blood plasma
- Total Triiodothyronine (Total T3 or TT3) – the total amount of T3, both unbound and bound to proteins, per unit volume of blood plasma
- Free Triiodothyronine (Free T3) – the amount of unbound T3 per unit volume of blood plasma
- Thyroxine-binding globulin (TBG) –the most important of three proteins responsible for carrying thyroid hormones in the bloodstream
- Thyroglobulin – a protein made by the follicular cells in the thyroid
- Transthyretin (TTR) – a serum and cerebrospinal fluid carrier of T4
- Albumin – the main protein of human plasma; binds to T4 and T3
- Thyroid Hormone Uptake – a measure of the unbound thyroxine-binding proteins in the blood
- Thyroid Hormone Binding ratio (THBR) – a measure of the “uptake” of T3 or T4 tracer by thyroid-binding proteins in a given serum sample
- Free Thyroxine Index – This test is rarely used now that reliable free thyroxine and free triiodothyronine assays are routinely available
- Thyroid Peroxidase Antibodies (TPO Ab) – antibodies that attack the thyroid peroxidase enzyme. The levels of these are raised in Hashimoto’s disease
- Thyroglobulin antibodies (TG Ab) – Antibodies that attack thyroglobulin. The levels rise as well as the TPO antibody levels in autoimmune thyroiditis, but to a lesser degree
Note that the TT3 and TT4 tests have largely been superseded by the FT4 and FT3 measurements. Only the free thyroid hormones are biologically active and they represent only about two ten-thousandth of total T4 and three-thousandths of total T3. This makes the assessment of FT4 levels from the TT4 levels very inaccurate.
Usually, the only test that is actually included in the thyroid function test is thyroid-stimulating hormone (TSH). If the TSH measurement is out of range then usually FT4, and sometimes FT3, will also be included in the thyroid function test panel (antibody tests are done far more often than the FT3 test).
In thyroid function tests the thyroid hormone levels measured in your blood sample are compared against ‘reference ranges’ (or levels) which are meant to represent the range of readings found in 95% of the whole healthy population. However, when looking at your thyroid test results it is really important to always check the reference levels, particularly for FT3 and FT4, as they often vary between different laboratories for the reasons described below.
Variation of Reference Ranges or Levels
Of all the assays, TSH has been the most rigorously standardised using international standards. This is because there are just so many TSH molecules in a serum sample – no more and no less.
For FT4 and FT3 this isn’t the case – these are the result of the ever-changing balance between bound and free hormone. You can say that there are a certain number of FT4 and FT3 molecules in an undiluted serum sample, but the moment you try to measure them the relationship between bound and free hormone changes. This is because an antibody probe has to be put in which itself binds to the hormone so affecting the natural balance.
The FT3 and FT4 measurements are therefore very difficult to standardise because rather fragile methods have to be relied upon. The ranges established by individual manufacturers using these inadequate methods often disagree significantly. Individual labs very often establish their own ranges because they believe that their particular population differs from the manufacturers. However, other labs just use the manufacturer’s suggested range.
Because of the disagreement over FT4, and FT3 especially, ranges are frequently derived by the laboratory, often using methods that leave a lot to be desired. There is also no control over what mix of subjects is used – patients or healthy staff. If the numbers are also too small, then the results become uncertain. Often labs rely on only 100 samples which is nowhere near enough to obtain a dependable result.
Clinical Validity of Reference Ranges
It should be borne in mind that although the TSH test is regarded by the medical establishment as the ‘Gold Standard’ test for hypothyroidism, the TSH test result alone does not necessarily correlate with symptoms. However, TSH is used as a criterion for including or excluding patient results when fixing the reference ranges for FT3 and FT4, where data from people with thyroid conditions should be excluded. Unfortunately, this use of the TSH test impairs the accuracy of the reference ranges for FT3 and FT4. It is also the case that TSH ranges obtained from healthy people do not apply to patients on thyroid hormone therapy and the relation between TSH and either FT4 or FT3 is altered.
Examples of Reference Ranges
The table below shows examples of approximate reference ranges and measurements used for the various tests. Note that reference levels/ranges may vary between laboratories.
|Measurement||Lower Limit||Upper Limit||Measurement Unit|
|Thyroid Stimulating Hormone (TSH)||0.4||4.5||μIU/mL|
|Total Thyroxine (TT4)||50||160||nmol/L|
|Free Thyroxine (FT4)||10||24||pmol/L|
|Total Triiodothyronine (TT3)||60||175||ng/dL|
|Free Triiodothyronine (FT3)||4||7.8||pmol/L|
The measurement units are expressed as the amount of the substance per unit volume of blood serum.
In the above examples, the amount of the substance is expressed as either:
- International Units (IU) (sometimes just written as U) or
- Moles (mol), a unit of measurement used in chemistry to express amounts of a chemical substance (in this case, thyroid hormone)
- Grams (g)
and the unit volume of blood serum containing the amount of the substance is expressed in litres (L).
Because of the very tiny amounts of the substance being measured prefixes are used as follows:
- d = Deci = tenth
- m = Milli = thousandth
- μ = Micro = millionth
- n = Nano = billionth
- p = Pico = trillionth
For example, the TSH measurement unit μIU/mL should be read as millionths (μ) of an international unit (IU) of the hormone (TSH) per thousandth (m) of a litre of blood plasma (L).
Some substances are measured by how much effect they have rather than the absolute amount in terms of weight or volume. This is often expressed using the International unit which is often referred to simply as Unit. The standard abbreviation is lU – though often simply U.
Very often, International Units were created before anyone knew what substance was having the effect. International Units are always strictly related to single substances. There is absolutely NO relationship between, say, an lU of vitamin D and an lU of vitamin A.
For vitamin D, one microgram is 40 international units whereas for vitamin A it varies greatly depending on which form of vitamin A you are talking about.