TSH 3rd Generation
About the test:
Thyroid-stimulating hormone (TSH) is produced by the anterior pituitary gland, a small organ located below the brain and behind the sinus cavities. TSH stimulates the thyroid, by binding to the TSH receptor to release the hormones thyroxine (T4) and triiodothyronine (T3) into the blood. This test measures the amount of TSH in the blood.
When thyroid hormone levels decrease in the blood, the pituitary gland produces more TSH in response to TRH stimulation. TSH in turn stimulates the thyroid to produce and release more T4 and T3.
When thyroid hormone levels increase in the blood, the pituitary gland produces less TSH, and the thyroid produces less T4 and T3.
When all three organs (hypothalamus, pituitary and thyroid) are functioning normally, thyroid production is regulated to maintain relatively stable levels of thyroid hormones in the blood.
If there is decreased production of thyroid hormones by the thyroid (underactive thyroid or hypothyroidism), the person may experience symptoms such as weight gain, dry skin, constipation, cold intolerance, and fatigue. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. It is a chronic autoimmune condition in which the immune response causes inflammation and damage to the thyroid as well as the production of autoantibodies. However, the autoantibodies do not cause the hypothyroidism. The detection of thyroid-related autoantibodies (e.g., thyroperoxidase autoantibodies and/or thyroglobulin autoantibodies) indicate that thyroid autoimmunity is present. These autoantibodies can be detected in Graves disease or Hashimoto thyroiditis. With Hashimoto thyroiditis, the thyroid produces low levels of thyroid hormone. In response, the pituitary normally produces more TSH, usually resulting in a high level in the blood.
However, the level of TSH alone does not always predict or reflect thyroid hormone levels. Some people with pituitary disease produce an abnormal form of TSH that does not function properly. They often have hypothyroidism despite having normal or even mildly elevated TSH levels.
Rarely, pituitary dysfunction may result in increased or decreased amounts of TSH. In addition to pituitary dysfunction, hyperthyroidism or hypothyroidism can occur if there is a problem with the hypothalamus (insufficient or excessive TRH).