What Do My Thyroid Hormone Blood Tests Tell?

The Answer

Thyroid blood tests measure the thyroid gland’s functioning. Different combinations of outcomes indicate various disorders.

The Science

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
Image Credit: stefamerpik on Freepik