Blood Tests for Thyroid Problems
The thyroid gland is a butterfly-shaped gland in the neck that controls the body's metabolism through a delicate feedback system, and any dysfunction in the feedback loop can cause thyroid disease. Blood tests are usually the first tests ordered for a patient with potential thyroid dysfunction. Blood tests can determine the adequacy of circulating thyroid hormones, whether the disease has an auto-immune component and whether thyroid dysfunction is due to cancer. Blood tests can measure the levels of TSH (thyroid stimulating hormone), T4 (thyroxine), T3 (triiodothyronine), thyroid auto-antibodies and other proteins circulating in the blood.
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Feedback System
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The pituitary gland releases TSH, which controls thyroid function. TSH stimulates the thyroid to uptake iodine and transfer it to thyroglobulin (Tg), creating T3 and T4. T3 and T4, collectively termed the thyroid hormones, circulate throughout the bloodstream and control metabolism. The pituitary gland functions as a thermostat, sensing levels of T3 and T4 in the body and releasing TSH appropriately. If the T3 and T4 levels are inadequate, the pituitary releases TSH to stimulate the thyroid's production of thyroid hormones, and vice versa. In turn, the pituitary gland is regulated by the hypothalamus.
TSH
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TSH is usually the first test ordered for a patient with potential thyroid dysfunction. If the pituitary secretes TSH to stimulate the thyroid to produce T4, and the thyroid does not produce T4 due to dysfunction, the pituitary will repeatedly secrete more and more TSH to signal the thyroid gland to produce T4. In this way, high TSH is indicative of primary hypothyroidism, where a dysfunction in the thyroid gland itself causes low T4 and hypothyroidism. The severity of hypothyroidism is directly related to the TSH level in the blood.
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T4
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T4 circulates in the blood as free T4 (unbound to any protein) and T4 bound to proteins such as TBG (thyroid binding globulin). The free T4 level, combined with the TSH level, are used to determine whether the thyroid gland is over functioning, functioning normally or under functioning. Low free T4, combined with high TSH, is present in hypothyroidism. Likewise, high T4, and low TSH, is present in hyperthyroidism.
T3
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The T3 test is most useful in diagnosing or determining the severity of hyperthyroidism, after a patient receives an abnormal TSH or T4 result. T3 can be elevated significantly or only slightly in hyperthyroidism.
Thyroid Peroxidase Antibody
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Thyroid peroxidase (TPO) is an enzyme essential to the production of T4 and T3 in the thyroid gland. Most people with Hashimoto's thyroiditis, an auto-immune thyroid disease and the most common kind of thyroid disease, have elevated levels of antibodies against TPO. These antibodies can also be present, but are not common, in other autoimmune diseases.
Thyroglobulin (Tg)
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Thyroid cancer often requires a complete thyroidectomy. Because thyroglobulin (Tg) is only produced by the thyroid gland, the test for Tg is helpful to determine the amount of thyroid gland present after a complete thyroidectomy. The amount of Tg in the blood is also helpful in determine the efficacy of cancer treatment.
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