The thyroid gland, a butterfly-shaped organ, is located in the front of the neck straddling the windpipe. The two lobes of the thyroid are connected by a thin strip of tissue called the isthmus. The thyroid is an endocrine gland, meaning it produces hormones that are secreted directly into your bloodstream. The hormones produced by the thyroid stabilize your metabolism, or the rate at which your body uses energy, and to a lesser degree, your calcium levels. It is responsible for setting your basal metabolic rate, which is measured in calories and describes the energy your body uses at rest.
How the Thyroid Works
The tissue of the thyroid contains two different types of cells. Most of its tissue is composed of follicular cells, which are responsible for secreting the iodine-containing hormones T3, or triiodothyronine, and T4, or thyroxine. The medical website Endocrine Web states that a healthy thyroid produces 80 percent T4 and 20 percent T3 hormones. The less numerous parafollicular cells secrete calcitonin, which works together with hormones from the parathyroid glands to control the levels of calcium in your blood. Your pituitary gland produces thyrotropin, also called thyroid stimulating hormone, which also affects thyroid function.
Thyroid Stimulating Hormone Levels
Your pituitary gland produces and releases thyroid stimulating hormone, or TSH, in order to regulate the amount of T3 and T4 hormones your thyroid gland produces. Levels of TSH are tested using a blood test. MedlinePlus states healthy levels of TSH are 0.4 to 4.0 mIU/L, or milli International Units per Liter. A low level of TSH is indicative of an overactive thyroid gland, or hyperthyroidism, and a high level can mean hypothyroidism, or an underactive thyroid gland. A TSH test is often performed prior to other thyroid hormone tests.
T4 Levels
A T4 blood test is used to evaluate how well your thyroid is functioning. There are two separate tests for T4 that may be used to determine this. The total T4 test that has been used for many years to evaluate thyroid function requires the hormone to bind to protein and therefore can be adversely affected by the amount of protein in the blood. A newer test which tests for free T4 does not require it to bind to protein in order to be measured and is considered more reliable. Endocrine Web states normal levels for total T4, or serum thyroxine, are 4.6 to 12.0 mcg/dL and for free T4 they are 0.7 to 1.9 ng/dL.
T3 Levels
Levels of T3 are also tested via a blood test for either protein-bound T3 in serum triiodothyronine or free T3. Healthy serum levels of T3 are 80 to 180 ng/dL and free T3 levels are 230 to 619 pg/dL, according to Endocrine Web. Increased levels of T3 or T4 typically indicate an overactive thyroid gland and low levels are associated with an underactive thyroid gland. Certain drugs can interfere with these tests, so your doctor will advise you what drugs to refrain from taking before you have your blood tested.
