The Thyroid Stimulating Hormone (TSH) blood test is often performed to determine how the thyroid gland is performing through the measurement of the amount of TSH in the blood. A common reason for ordering a TSH blood test is to determine whether a patient has hyperthyroidism or hypothyroidism. Hyperthyroidism means that the thyroid gland, which is found in front of the windpipe, produces too much of the thyroid hormones such as triiodothyronine (T3) and thyroxine (T4). Hypothyroidism means that there is a lack of these hormones in the body. A person with hypothyroidism usually has symptoms such as weight gain, frequent menstruation, constipation, fatigue and dry skin. On the other hand, hyperthyroidism can cause a person to have symptoms like weight loss, fast heart beats, irregular menstruation, nervousness, and diarrhea.
The TSH blood test may be ordered for people who have goiter or an enlarged thyroid gland and those who have symptoms indicative of hyperthyroidism or hypothyroidism. It is the hypothalamus that produces the thyrotropin-releasing hormone (TRH) that in turn causes the pituitary gland to produce TSH. It is TSH that controls the production of T3 and T4 by the thyroid gland. Normally, if the pituitary gland senses a low level of T3 and T4 in the blood, it releases TSH to cause the thyroid to release more of these hormones.
To help in the interpretation of the results of the TSH blood test, a physician usually requires also the measurement of the levels of T3 and T4 in the patient's blood. Thus, if the patient has unusually high levels of TSH but has low quantities of the T3 and T4 hormones and vice versa, it can be determined that it is a thyroid problem. However, if both the thyroid hormones and TSH are too high or both are too low, then it is the pituitary gland that may be diseased.
However, some medication or drugs, such as aspirin, can affect the level of TSH in the blood. During the first stages of pregnancy, TSH levels may also be low. Other conditions, such as too much stress, exposure to radioactive materials or an x-ray using iodine dye, can also influence the amount of TSH in the blood.