Blood Test for Thyroid Diagnostics

A blood test is usually the first test conducted to monitor thyroid function and the production of thyroid hormones: T3, T4, and thyroid stimulating hormone (TSH), the hormone that stimulates the thyroid gland to produce thyroid hormone. TSH is the most sensitive index of thyroid function. Low TSH indicates that the thyroid is producing too much hormone while an elevated TSH means too little thyroid hormone is being produced. Therefore, high levels of thyroid hormones and a low TSH indicate hyperthyroidism while low levels of thyroid hormones and a high TSH are indicative of hypothyroidism.

A blood test can measure calcitonin, a protein made by special cells of the thyroid gland called C-cells. When these cells become malignant and develop into a medullary carcinoma of the thyroid, they can produce large amounts of calcitonin, which is used diagnostically and as a tumor marker.

In addition, measurements of thyroglobulin may be performed. Such a measurement is very useful in monitoring individuals who have had their thyroid removed for thyroid cancer since it is an excellent tumor marker for papillary and follicular cancers of the thyroid gland.

If an autoimmune thyroid condition is suspected (Hashimoto's thyroiditis) then anti-thyroid peroxidase and antithyroglobulin antibody measurements in the blood may be made to confirm the diagnosis.