What are thyroid hormones? What does the thyroid look like?
The thyroid gland produces hormones that are essential for normal body metabolism. Blood testing is now commonly available to determine the adequacy of the levels of thyroid hormones. These blood tests can define whether the thyroid gland's hormone production is normal, overactive, or underactive.
The thyroid gland is located in the lower part of the neck, below the Adam's apple.The gland wraps around the windpipe (trachea) and has a shape that is similar to a butterfly - formed by two wings (lobes) and attached by a middle part (isthmus).
The thyroid gland uses iodine (mostly available from the diet in foods such as seafood, bread, and salt) to produce thyroid hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3), which account for 99.9% and 0.1% of thyroid hormones present in the blood respectively. However, the hormone with the most biological activity is T3. Once released from the thyroid gland into the blood, a large amount of T4 is converted into T3 - the active hormone that affects the metabolism of cells.
What are normal thyroid hormone levels?
The thyroid itself is regulated by another gland that is located in the brain, called the pituitary. In turn, the pituitary is regulated in part by the thyroid (via a "feedback" effect of thyroid hormone on the pituitary gland) and by another gland called the hypothalamus.
The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a signal to the pituitary to release thyroid stimulating hormone (TSH). In turn, TSH sends a signal to the thyroid to release thyroid hormones. If a disruption occurs at any of these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone (hypothyroidism).
Hypothalamus - TRH
Pituitary - TSH
Thyroid - T4 and T3
The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid hormone production. In contrast, when there is an excessive amount of circulating thyroid hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid hormone. In persons with hypothyroidism (thyroid hormone production is below normal), there is a continuously decreased level of circulating thyroid hormones. In persons with hyperthyroidism (thyroid hormone production is above normal), there is a continuously elevated level of circulating thyroid hormones.
What procedures and tests diagnose thyroid problems?
Blood tests are used to diagnose hypothyroidism or hyperthyroidism. They do not point to a specific cause. In order to determine a cause of the thyroid abnormality, the doctor will consider the patient's history, physical examination, and medical condition. Further testing might be used to isolate an underlying cause. These tests might include more blood testing for thyroid antibodies, nuclear medicine thyroid scanning, ultrasound of the thyroid gland, or others.
If thyroid cancer is suspected and surgery may be required, your physician may ask for a blood test known as thyroglobulin. Thyroglobulin is a protein made only by thyroid cells. If the thyroglobulin level at baseline is detectable or elevated (this means the gland does in fact make the protein) it can be used as a tumor marker. After a total thyroidectomy for cancer (removal of the entire thyroid gland) the level should fall to an undetectable range since the cells that make thyroglobulin have been removed. If the level remains detectable after surgery, there is a possibility of thyroid tissue elsewhere in the body, and metastatic disease should be considered. If the level is undetectable for a period of time after surgery and then starts to climb, a recurrence of the cancer - either at the primary site or elsewhere in the body should be considered.
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Medically reviewed by John A. Seibel, MD; Board Certified Internal Medicine with a subspecialty in Endocrinology & Metabolism.
National Endocrine and Metabolic Diseases Information Service (NEMDIS).