The most common thyroid function blood test measures the levels of thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4).
Thyroid stimulating hormone (TSH)
TSH is made by your pituitary gland, which is located in your brain. The pituitary senses the level of thyroid hormones in your bloodstream, just as the thermostat in your living room senses the temperature.
TSH is your pituitary gland’s messenger. If the level of thyroid hormones T3 and T4 drop just a little below normal, the pituitary reacts by secreting TSH, which tells your thyroid gland to produce more or less T3 and T4.
In healthy people, this system regulates itself perfectly. However, if you have a thyroid or pituitary condition the system gets unbalanced.
Triiodothyronine (T3) and thyroxine (T4)
T3 and T4 are produced by your thyroid gland. T3 makes up less than 10% of what is called thyroid hormone, but it is potent and is thought to cause most, if not all, of the effects of thyroid hormones. T4 makes up nearly all of what is called thyroid hormone but is thought to have less effect.
Usually, the ‘free’ or ‘active’ portions of T3 and T4 are measured, so these tests are known as FT3 and FT4.