A blood test is usually the first test conducted to monitor thyroid function and the production of thyroid hormones. TSH is the most sensitive index of thyroid function and stimulated the thyroid gland to produce thyroid hormone. 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. 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. Other thyroid hormones that can be measured in the blood include: T3, T4, thyroid peroxidase (TPO), thyroid antibody (Tgab), TSH receptor antibody, and thyroid binding immunoglobulin.
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.
For the rare medullary carcinoma case, calcitonin can be measured. It is a protein made by special cells of the thyroid gland called C-cells. When these cells become malignant, they can produce large amounts of calcitonin. This blood test is used diagnostically and as a tumor marker for patients with medullary thyroid carcinoma.
Your endocrinologist will work closely with your particular case to determine which blood tests are right for you.