What are Thyroid Function Tests?
Thyroid function tests are blood tests used to check for thyroid problems or whether your thyroid is functioning normally.
The thyroid is a small, butterfly-shaped gland located in front of your neck just below the voice box (larynx). It secretes hormones that are instrumental in the proper functioning of various body parts such as the brain, heart, and muscles, and helps regulate various body functions such as metabolic rate, heart rate, breathing, body temperature, weight, and blood pressure. Any abnormality of this gland can cause many physiological and psychological problems in the body.
Thyroid function tests are typically used to detect an overactive thyroid gland (hyperthyroidism) and an underactive thyroid gland (hypothyroidism). The tests measure the level of thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) in your blood. TSH is typically tested first. The combination of a TSH test and tests for the thyroid hormones T3 and T4 is known as a thyroid function test (TFT). Additional thyroid tests include thyroid antibodies.
Indications for Thyroid Function Tests
Your physician may recommend thyroid function tests if you have symptoms of hyperthyroidism or hypothyroidism.
Signs and symptoms of hyperthyroidism include:
- Weight loss
- Sensitivity to heat
- Nervousness
- Fast heartbeat
- Sweating
- Increased stool frequency (pooing)
Signs and symptoms of hypothyroidism include:
- Fatigue or tiredness
- Weight gain
- Weakness
- Muscle aches
- Slow heart rate
- Constipation
- Sensitivity to cold
Thyroid function tests may also be recommended to:
- Monitor treatment with thyroid replacement medicine for people who have hypothyroidism
- Check thyroid gland function in people who are being treated for hyperthyroidism
- Screen newborn babies for inherited problems with the thyroid gland
- Check for infertility in women
- Diagnose other thyroid diseases like thyroiditis, Graves’ disease, and Hashimoto’s disease
Preparation for Thyroid Function Tests
Thyroid function tests normally require very little preparation. Tell your physician if you are taking any medications, especially medications for thyroid disorders, as some medications can alter the test results and how they are interpreted. Also, tell your physician if you are pregnant as levels of thyroid hormones can change during pregnancy.
What Happens During a Thyroid Function Test?
A thyroid function test is a simple blood test performed at the medical office or clinic. When you arrive at the medical office or clinic, you will be seated in a comfortable chair. Your healthcare provider or a nurse will check your upper arms for an easily accessible vein. This is usually in the inner part of your upper arm on the other side of your elbow. Once they have located a vein, an elastic band is wrapped around your upper arm to make the vein swell with blood. The site of needle insertion at the vein is cleaned with an antiseptic solution. A small needle is then inserted into your vein and the required sample of blood is drawn out. You may feel a slight stinging sensation when the needle goes in and out of your skin. The extracted blood sample is collected in a vial or test tube and preserved for laboratory analysis thereafter. A cotton gauze is applied over the insertion site as the needle is withdrawn to control bleeding. A small bandage is placed over the insertion site after the procedure is complete. The entire process takes around five minutes or less.
How Do Thyroid Function Tests Work?
Thyroid blood tests measure certain thyroid hormones and antibodies in your blood. Too many or too few of these hormones or antibodies may indicate that you have a thyroid disorder. The usual blood tests done for thyroid function are TSH, T4, and sometimes T3. In most cases, your TSH level will be the first hormone to be measured, because if this is normal, it is very likely that your thyroid is functioning normally. The exception to this is when a pituitary problem is suspected, in which case T4 should be measured as well.
Providers use different blood tests to measure different things:
- TSH measures your thyroid-stimulating hormone. This is usually the first test most providers recommend. Your pituitary gland (part of your brain) makes TSH. It travels to your thyroid gland, stimulating it to produce thyroid hormones, T3 and T4.
- T3 (or free T3) measures the amount of triiodothyronine in your blood. This hormone is one of the two main hormones that your thyroid makes.
- T4 (or free T4) measures the amount of thyroxine in your blood. Thyroxine is the other main type of hormone that your thyroid makes.
- Thyroid antibody tests inform your provider whether or not there are thyroid antibodies in your blood. The presence of thyroid antibodies might mean you have an autoimmune disorder like Graves’ disease or Hashimoto’s disease.
Results of Thyroid Function Tests
Many laboratories employ a ‘cascade’ system, so the other hormones will be measured if the TSH is not normal.
- If the TSH level is below the reference range, the blood T4 and T3 will be measured.
- If the TSH level is above the reference range, the blood T4 will be measured.
Typical reference ranges for healthy adults are:
- Normal range for TSH: 0.4-4.0 mU/l (milliunits per liter)
- Normal range for T4: 9-24 pmol/l (picomoles per liter)
- Normal range for T3: 3.5-7.8 pmol/l (picomoles per liter)
So, in general, if you have a low TSH level and a high T4 level, you may have an overactive thyroid gland. Conversely, if you have a high TSH level and a low T4 level, you may have an underactive thyroid gland. There are other alternatives such as having a high TSH level but a normal T4 level and this can indicate something called subclinical hypothyroidism or mild thyroid failure. It may gradually develop into full-blown or clinical hypothyroidism over many years; an additional test for thyroid antibodies will assist in determining the risk.
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