Thyroid blood tests measure the thyroid gland’s functioning. Different combinations of outcomes indicate various disorders.
The thyroid gland is a butterfly-shaped gland in the front portion of your lower neck. It makes hormones that help your body produce energy, regulate temperature, and allow your vital organs to function properly.
The thyroid gland secretes two main hormones—twenty per cent is triiodothyronine (T3) and eighty per cent is thyroxine (T4). The former is the active hormone used by the body, while the latter needs to be converted into T3 by the body before usage.
A gland called the pituitary helps control the thyroid gland by secreting a hormone named Thyroid Stimulating Hormone (TSH). The pituitary gland senses blood levels of T3 and T4 and if they are low, releases TSH to tell the thyroid gland to make more T3 and T4, and vice versa.
- The first and the most basic thyroid test is for TSH levels. If TSH is normal, the thyroid is functioning properly in most individuals. If TSH levels are not normal, you need to measure T4 levels.
- If TSH is low, it usually means the thyroid is overactive and produces too much T4, making the pituitary go easy on TSH production, indicating hyperthyroid.
- If TSH levels are high, it means the thyroid gland needs some stimulation to produce T4, indicating an underactive or hypothyroid.
- It is important that your doctor interprets your test results and not you because sometimes, high or low T4 levels may not indicate a thyroid problem. Here is why:
- Most T3 and T4 are attached to some proteins in the blood. They are called bound T3 and bound T4, respectively, and are effectively the body’s reserves.
- Free T3 and T4 are unbound and can be used by the body immediately.
- The measured values of T3 and T4 include both the bound and free levels.
- Pregnant women or those on oral contraceptives may have high T4, while steroid medicines for asthma, arthritis, and skin problems can lower T4 levels. These conditions change the amount of blood proteins that bind to T3 and T4, changing the measured T3 and T4 levels. But free T3 and T4 levels don’t change.
- If your doctor feels these may be distorting your T4 results, she may ask for measuring free T4 instead of total T4. Blood levels of free T4 are fifty times lesser than those of total T4 and so unless needed, measuring total T4 is a better option.
- T3 levels are mainly useful in diagnosing hyperthyroidism because in some patients, T4 may be normal and T3 high.
- Free T3 is neither reliable nor very useful.
To Read More
- National Institute of Diabetes and Digestive and Kidney Diseases: Thyroid Tests
- American Thyroid Association: Thyroid Function Tests
First Published on: 7th May 2023
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