Thyroid Function Tests
Blood tests for thyroid disease can be broken down into tests of thyroid function (i.e. how much thyroid hormone is being made), thyroid mass (i.e. how much thyroid tissue there is), and antibodies to thyroid tissue.
Tests of thyroid function:
- TSH (thyroid stimulating hormone or thyrotropin):
The pituitary gland makes TSH to stimulate the thyroid to make and release more thyroid hormone. When there is enough or too much thyroid hormone in the body, the TSH level drops. When there is too little thyroid hormone in the body, the TSH level rises. Therefore, if a patient is hyperthyroid (i.e. has too much thyroid hormone), theirTSH level should be low or "suppressed" because the brain does not want to stimulate the thyroid to make any more thyroid hormone. If a patient is hypothyroid (i.e. has too little thyroid hormone), there TSH level should be high because the brain is trying to get the thyroid to make more thyroid hormone.
- Free T4 (thyroxine):
T4 is the most common form of thyroid hormone in the body. Over 99% of thyroid hormone is attached to another protein in the blood, measuring the amount of unattached or "free" thyroid hormone is a much more accurate measurement of thyroid hormone levels. If the T4 level is higher than normal, the patient may be hyperthyroid. If the T4 level is lower than normal, the patient may or may not be hypothyroid. Synthroid or Levothyroxine is the man-made form of T4 that can be given to replace a person's own thyroid hormone.
- T3 (triiodothyronine):
T3 is the less common form of thyroid hormone in the body. For this reason, it is typically only measured in cases of hyperthyroidism in which the T4 levels are normal.
Tests of thyroid mass:
- Thyroglobulin (Tg):
Thyroglobulin is a protein made by the thyroid that attaches to thyroid hormone to help it move throughout the body. Thyroglobulin is only made in the thyroid and almost all thyroid cells will make it, even thyroid cancer cells. For this reason, thyroglobulin is a good tumor marker (i.e a blood test that tells you how much of a particular cancer is in the body) if all of the normal thyroid tissue has been removed. If the whole thyroid is removed (i.e. total thyroidectomy), the Tg level should be close to zero. If the levels then go up, then the thyroid cancer may have come back.
- Anti-thyroglobulin antibodies: Some patients may make antibodies to thyroglobulin called anti-thyroglobulin antibodies. These antibodies may make the thyroglobulin levels less accurate as a tumor marker.
Calcitonin is made by a specific type of thyroid cell called the parafollicular cell and is used as a tumor marker for a rare type of cancer called medullary thyroid cancer.
Antithyroid antibodies are antibodies that attack parts of the thyroid. These antibodies are part of an autoimmune response in which the body attacks specific parts of itself.
- TSH-receptor antibodies:
The TSH-receptor is the part of the thyroid cell that TSH attaches to in order to cause the thyroid to make thyroid hormone. Unlike most antibodies which destroy their targets, TSH-receptor antibodies actually stimulate it and cause the thyroid to make thyroid hormone. This in turn can lead to hyperthyroidism.
- Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies:
These antibodies cause the destruction of thyroid cells and may cause hypothyroidism. These antibodies are found in Hashimoto's thyroiditis.