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Background: Thyroid tests

1. Overview

The two most commonly used laboratory evaluations to test thyroid function are free T4T4 - thyroxine thyroid hormone and thyroid stimulating hormone (TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone). Because over 99% of released thyroid hormone is attached to proteins in the blood, it is believed that measuring the amount not attached to proteins (i.e., free thyroid hormone) is a more accurate measurement of thyroid hormone levels. Measuring the total hormone level is generally less reliable.

Common thyroid function tests

AssayHigh levelLow level
TSHHypothyroidismHyperthyroidism
Free T4HyperthyroidismHypothyroidism
Free T3 HyperthyroidismHypothyroidism
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

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2. Thyroxine (T4)

Typically measured as free hormone. Greater than 99.9% is bound to protein. A higher than normal level is associated with hyperthyroidismHyperthyroidism - overactive thyroid. A lower than normal level is associated with hypothyroidismHypothyroidism - underactive thyroid.

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3. Triiodothyronine (T3)

Measurements of T3 levels are not routinely performed. It is similar to T4T4 - thyroxine thyroid hormone in that over 99% is bound to proteins in the blood. T3 is typically only measured in cases of hyperthyroidismHyperthyroidism - overactive thyroid where the T4 levels are normal.

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4. Thyroid stimulating hormone (TSH)

Thyroid stimulating hormone (TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone), also known as thyrotropin, is produced by the pituitary gland in the brain and causes the thyroid to make thyroid hormone (T4T4 - thyroxine thyroid hormone and T3). The pituitary gland acts like the thermostat in your home and controls how much thyroid hormone is made by making more or less TSH. When there is not enough thyroid hormone in the body, the pituitary makes more TSH, which tells the thyroid 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.

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5. Thyroglobulin (Tg)

Thyroglobulin is a protein made by the thyroid that carries thyroid hormone. The thyroid gland is the only organ which makes TgTg- thyroglobulin; protein made by the thyroid that carries thyroid hormone. Therefore, Tg is a good test to tell if there are thyroid cells in the body. Once the whole thyroid is taken out, the thyroglobulin level should go down to close to zero. Thyroglobulin levels can then be measured, by a simple blood test, after 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 whole thyroid, there is concern that the cancer may have come back (i.e. recurred) or spread (i.e. metastasized).

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6. Antithyroid antibodies

Antithyroid antibodies are created when the body's immune system attacks parts of the thyroid with antibodies. There are several different types of antithyroid antibodies. TSH-R AbTSH-R Ab - antibodies to the TSH receptor; present in Graves' disease is a TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone receptor antibody, and is present in most of patients who have Graves' diseaseGraves' disease - autoimmune overproduction of thyroid hormone resulting in hyperthyroidism. (See Hyperthyroidism section) The antibodies stimulate the TSH-receptor which causes the thyroid to make more thyroid hormone (T4T4 - thyroxine thyroid hormone and T3).

Antithyroglobulin antibodies (Anti-TG AbAnti-TG Ab - Antithyroglobulin antibody; seen in Hashimoto's thyroiditis) are present in patients with Hashimoto's thyroiditis (See Hypothyroidism section). Antibodies against the protein thyroglobulin can result in destruction of thyroid cells. This destruction can lead to hypothyroidismHypothyroidism - underactive thyroid.

Antithyroid peroxidase antibodies (Anti-TPO AbAnti-TPO Ab - Antithyroid peroxidase antibody; seen in Hashimoto's thyroiditis ATC - Anapalstic thyroid cancer ) are similar to antithyroglobulin antibodies. They are found in high levels in patients with Hashimoto's thyroiditisHashimoto's thyroiditis - autoimmune thyroiditis causing hypothyroidism, and can lead to destruction of thyroid cells.

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7. Calcitonin

CalcitoninCalcitonin - hormone, secreted by C-cells, which plays a role in bone formation and deposition of calcium; can be used in the diagnosis and as a marker of recurrent disease in patients with MTC. 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)

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