The most commonly used clinical thyroid function measurements such as T3, T4, and TSH measurements are important for the diagnosis of various thyroid disorders. No laboratory test can diagnose a particular type of thyroid disease with 100% accuracy, but combining 2 or more tests can usually measure even minimally abnormal thyroid function. As mentioned earlier, the thyroid gland produces T4 and T3, but this production must be stimulated by TSH in the pituitary gland, which in turn is regulated by TRH in the hypothalamus. Knowing this can be very helpful in determining the condition. For example, low T4 levels can indicate thyroid disease or a non-functioning pituitary gland (unable to stimulate the thyroid gland to produce T4). Normally, a lower T4 results in the release of TSH from the pituitary gland, and if TSH levels are high, the thyroid gland (not the pituitary gland) is underactive. If T4 is low and TSH is not high, the lesion may be in the pituitary gland. Since the pituitary gland also regulates other glands in the body (adrenal glands, ovaries and testes), controls growth and normal kidney function in children, it can have a significant impact on treatment. Pituitary gland malfunction means that other glands may also be malfunctioning, and treatment of these gland malfunctions is more important than that of the thyroid gland. The most common cause of pituitary gland derangement is a tumor of the pituitary gland, which must be surgically removed. In general, low levels of TSH are sufficient to maintain normal thyroid function. In hypothyroidism such as early hypothyroidism, TSH rises, even though T4 and T3 remain within normal limits. This rise reflects the pituitary gland’s response to a decrease in circulating blood thyroid hormone and is usually the first sign of a thyroid disorder. TSH is usually low in normal thyroid function, and low circulating thyroid hormones with abnormally elevated TSH indicate an impaired pituitary gland. the interpretation of TSH levels depends on the level of thyroid hormones, so TSH measurements are usually combined with other thyroid tests such as T4 and T3. T4 is the most commonly used of all thyroid tests and reflects the amount of thyroid hormones in the blood. T4 is generally a good test of thyroid function if the patient is not taking any thyroid medications, and although T3 is only 20%, some patients have normal T4 and high T3 levels. Therefore, the combination of the two measurements provides a more accurate assessment of thyroid function.