What does thyroid function testing usually include?

Thyroid function testing typically involves assessing the levels of thyroid hormones to evaluate how well the thyroid gland is functioning. The primary tests included are Thyroid Stimulating Hormone (TSH) and free thyroxine (free T4).

TSH is produced by the pituitary gland and regulates the production of thyroid hormones by the thyroid gland. When the levels of T4 in the body are low, TSH levels will typically be elevated as the pituitary gland attempts to stimulate the thyroid. Conversely, if T4 levels are high, TSH levels will be low, indicating that the thyroid is overactive.

Free T4 is measured to determine the amount of thyroxine that is not bound to proteins in the blood and is available for use by the body. This is crucial because the bound form does not perform any physiological functions.

Sometimes, total or free T3 (triiodothyronine) is also measured, especially if there are clinical indications such as suspecting hyperthyroidism, as T3 can sometimes provide additional information about thyroid function.

In contrast, the other choices do not align with the standard components of thyroid function testing. For example, measuring creatinine and potassium relates to kidney function and electrolyte balance,