What Is The Thyroid?

In depth reading

The thyroid gland is one of the body’s most important endocrine organs because it helps to regulate the body’s metabolism.  The thyroid is located in the central neck and overlies the trachea (windpipe) and is below the thyroid cartilage (Adam’s apple) and cricoid cartilage. Although its size can vary, based on a person’s size and iodine levels in their diet, the thyroid generally weighs approximately 15-20 grams (1/2 – 3/4 oz). It is composed of a right and left lobe. The right and left lobes are connected by a thin strip of thyroid tissue called the isthmus. Adjacent structures include the parathyroid glands (which regulate the calcium levels in the body), the recurrent laryngeal nerves (which move the vocal cords), the esophagus (food pipe), and the carotid artery (within the carotid sheath), which is the main blood supply to the thyroid gland.

The thyroid takes iodine from the diet to make thyroid hormone (thyroxine). Thyroid hormone has a number of different effects. Thyroid hormone increases the heart rate and how forcefully the heart squeezes. It also affects how fast food moves through your GI tract, regulates bone loss, and how fast glucose is made and used up in the body. All of these effects are increased in patients who have overactive thyroid glands (hyperthyroidism) and decreased in underactive thyroid glands (hypothyroidism).

Thyroid hormone is stored in the thyroid gland and released into the blood stream as needed by the body. The release of thyroid hormone is controlled by the pituitary gland which makes a hormone called thyroid stimulating hormone (i.e. TSH) that stimulates the thyroid to make and release more thyroid hormone. When there is enough or too much thyroid hormone in the body, the TSH level goes down. (Figure 2)

Overview

The two most commonly used laboratory evaluations to test thyroid function are free T4 and thyroid stimulating hormone (TSH). Since greater than 99% of released thyroid hormone is bound by protein and, thus, non-function in the blood, it is believed that measuring the amount of thyroid hormone not bound by protein (i.e., free thyroid hormone) is a more accurate measurement of thyroid hormone levels rather than measuring the total hormone level since this value is generally less reliable.

Common thyroid function tests

AssayHigh levelLow level
TSHHypothyroidismHyperthyroidism
Free T4HyperthyroidismHypothyroidism
Free T3HyperthyroidismHypothyroidism
TgMay indicate cancer recurrence after total thyroidectomyAdequate treatment after removal or ablation of thyroid
TSH-R AbGraves’ disease 
Anti TG AbHashimoto’s thyroiditis 
Anti TPO AbHashimoto’s thyroiditis 
CalcitoninMedullary thyroid cancer 


Thyroxine (T4)

Typically measured as free hormone. Greater than 99.9% is bound to protein. A higher than normal serum level of free T4 hormone is associated with hyperthyroidism. A lower than normal level is associated with hypothyroidism.

Triiodothyronine (T3)

Measurements of T3 levels are not routinely performed. It is similar to T4 in that over 99% is bound to proteins in the blood. T3 is typically only measured in cases of hyperthyroidism where the T4 levels are normal.

Thyroid stimulating hormone (TSH)

Thyroid stimulating hormone (TSH), also known as thyrotropin, is produced by the pituitary gland in the brain and causes the thyroid to make thyroid hormones (T4 and T3). The pituitary gland controls how much thyroid hormone is made and released by making more or less TSH. When there is not enough thyroid hormone circulating in the body, the pituitary makes more TSH, which tells the thyroid gland to make and release more thyroid hormone. If there is too much thyroid hormone in the body, the pituitary makes less TSH which tells the thyroid to make and release less thyroid hormone. Therefore, when patients are hyperthyroid (overactive thyroid), the TSH level is low and when they are hypothyroid (underactive thyroid), the TSH level is high.

Thyroglobulin (Tg)

Thyroglobulin (Tg) is a protein made by the thyroid that carries thyroid hormone. The thyroid gland is the only organ which makes Tg. Therefore, Tg is a good test to tell if there are any thyroid cells in the body. Once the entire thyroid gland is removed with surgery, the thyroglobulin level should go down to close to zero. Thyroglobulin levels can then be measured, by a simple blood test, after a thyroidectomy. Thyroglobulin is used as a tumor marker (i.e. test to see how much cancer is in the body) for thyroid cancer. If thyroglobulin levels go up after removing the entire thyroid gland, there is concern that the cancer may have come back (i.e. recurred) or spread (i.e. metastasized).

Antithyroid antibodies

Anti-thyroid antibodies are antibodies that the immune system has generated against the thyroid gland.  There are several different types of anti-thyroid antibodies. Thyrotopin receptor (TR) antibody (Ab) is an antibody which binds the TSH receptor and its binding stimulates the thyroid gland to become overactive (hyperthyroidism) and make and release more thyroid hormone (T4 and T3).  Elevated TR  Abs are present in most of patients who have Graves’ disease. (See Hyperthyroidism section).  Anti-thyroglobulin antibodies (Anti-TG Ab) are Abs against the protein thyroglobulin, and its binding to thyroglobulin can kill thyroid cells through antibody dependent cell mediated cytotoxicity (ADCC), resulting in an underactive thyroid gland, or hypothyroidism. Anti-TG Abs are present in patients with Hashimoto’s thyroiditis (See Hypothyroidism section). Anti-thyroid peroxidase antibodies (Anti-TPO Ab) are similar to anti-thyroglobulin antibodies. They are found in high levels in patients with Hashimoto’s thyroiditis, and again can lead to destruction of thyroid cells through ADCC.

Calcitonin

Calcitonin is produced by the parafollicular, or C cells, found in the thyroid gland. This test is used as a tumor marker for a rare type of thyroid cancer known as medullary thyroid cancer. (See Medullary thyroid cancer section)