Laboratory tests for thyroid disorders need to be carried out to:
1. Check if your thyroid is working normally
2. Find the fault, if any
3. To tell whether the problem is mild or serious
4. To help find the correct dosage of any medication
5. Monitor progress
However, no test is 100% accurate. The tests used are:
FT4 - Free Thyroxine - Reference Range 9.0 - 24 pmol/L
FT3 - Free triiodothyronine - Reference Range 2.5 - 5.3 pmol/L
RT3 - Reverse T3
TSH - Thyroid-stimulating hormone – Reference Range 0.5 - 4.7 mIU/L
TPO - Thyroperoxidase
Tgab - Thyroglobulin antibodies
T4 - measures the amount of T4 in the blood, both free and attached to proteins. If the reading is below 9 standard units, it suggests an underactive thyroid; if it is more than 24 standard units, this could mean an overactive thyroid, but other things may not be equal. A shortage of carrier protein will give a low result and an excess of protein will give a high result. Even so, the thyroid may be working correctly.
Increased protein and hence a high reading may be caused by many problems:
- Pregnancy, HRT (or contraceptive pill)
- Hepatitis
- Porphyria
- Cannabis
- hereditary
Reduced protein giving a low reading could be caused by:
- Steroids used in illness
- Bodybuilding steroids
- Nephrosis
- Cirrhosis
- hereditary
To check if an abnormally high or low T4 level is due to the thyroid, there are a couple of choices - the TSH or the FT4 test.
TSH TEST
This is often used instead of, rather than as well as the T4 test. If
the thyroid is not producing enough hormone for the body's requirements.
TSH comes into action to make the thyroid increase production. If the
TSH result is above the critical level (2 or more in either international
or standard units), the thyroid is underactive.
FT4 TEST
Some medical practitioners believe that this is the true measure of thyroid
activity - other practitioners do not believe this test should be used
alone, without taking into account clinical examination and symptoms
TPO and Tgab are tests for thyroid antibodies produced by the system itself, and above normal levels indicate autoimmune thyroiditis (Hashimoto's disease).
T3, FT3 AND RT3 TESTS
A high FT3 level occurs in T3 hyperthyroidism or Graves Disease. With
a healthy thyroid, increasing age causes a slow reduction in T3, unlike
T4 and a number of illnesses have the same effect - low T3 syndrome. Going
on a fast, unwanted starvation and anorexia nervosa all induce a low T3
level with a corresponding increase in reverse-T3.
ANTIBODY TESTS
From a patients point of view, these are simple - just an ordinary blood
test. For the laboratory, they are complex. Antibodies in the blood indicate
susceptibility to autoimmune thyroid problems.
1. Graves' disease. The main responsible antibody is TRAb - thyroid receptor antibody. If there is a high level of this during pregnancy, it is a warning to take action to protect the unborn baby
2. Hasimoto's disease. Several antibodies are involved, including anti-Tg (antithyroglobulin) and anti-M (antimicrosomal). The majority of Hashimoto suffers carry these antibodies, but so do 1 in 5 people who have never had a thyroid problem. This last group may, however, be more susceptible to developing an autoimmune disorder if the thyroid is put under stress, by, for instance, lithium medication, an infection or a faulty diet.
24-HOUR
URINE THYROID HORMONE TESTING
The 24-hour urine thyroid test serves as a valuable tool for detecting
thyroid dysfunction that can go undetected through the usual standard
blood tests. It is important to use this test in conjunction with other
indicators of thyroid function, such as body temperature, symptoms and
standard blood thyroid tests.
RADIOACTIVE UPTAKE (RAIU) TEST
This tests how effectively the thyroid cells are latching on to the iodine
in the circulation, which is a necessary ingredient of thyroid hormones.
The test starts with a scan of your basic level of radioactivity, with
a sort of Geiger counter. They you are given a measured dose of a mildly
radioactive form of iodine in a capsule or as a liquid. The thyroid area
is then scanned again at various intervals up to 24 hours to see how much
of the iodine has been taken up. For a quicker test the follow-up scan
can be done three to four hours after the start, but in this case you
must do without food during the whole time.
The results are useful in diagnosis and also ion assessing the dosage necessary if radioiodine treatment is in view.
High uptake will result from:
- Graves disease and other overactivity
- Iodine deficiency
- Having stopped antithyroid drugs
- A diet full of soya
- Kidney disease
Low uptake will result from:
- An underactive thyroid
- Medication containing iodine
- Diet - 'iodine-enriched foods or vitamins products
- Taking thyroxine - you must stop one month before the test
- Previous radioiodine treatment or thyroid operation
- Old age
- Having just exercised very energetically
The radioactive iodine used for the RAIU test has nothing like the strength of that used in treatment. Its radioactivity only lasts for ¾ days. Another radioactive material, technetium, is sometimes used instead of iodine; it is given by injection. Whichever material is used, the test is unsuitable for young children or anyone who might be pregnant, even at this low level of radiation.