Characteristic symptoms and physical signs can suggest that hyperthyroidism may be present; however, laboratory evaluation is necessary to establish the diagnosis and cause of hyperthyroidism.
Diagnostic lab tests performed on a blood sample include:
- Thyroid stimulating hormone (TSH)
- TSH level will be low in hyperthyroidism
- TSH assay is the most sensitive test for diagnosis of hyperthyroidism
- Free T4 (free thyroxine)
- The free or unbound thyroid hormone in the blood will be high in hyperthyroidism
- In patients with unstable thyroid states, T4 levels are sometimes more accurate than TSH as indicators of thyroid status
- With mild hyperthyroidism, the free T4 will remain in the normal range.
- Triiodothyronine (T3) radioimmunoassay (RIA) or free T3
- This form of thyroid hormone is 20 to 50 times more biologically active than T4
- T4 is converted in many organs (i.e. liver, kidneys) to the more bioactive T3 with the removal of an iodine by an enzyme called a deiodinase
- T3 is often elevated to a relatively higher level than T4 in severe hyperthyroidism.
- Thyroxine (T4)
- Total T4 in the blood measures both free and bioactive protein-bound T4
- Thyroid autoantibodies: TSH receptor antibodies (TRAb) or thyroid-stimulating immunoglobulins (TSI)
- These antibodies are present in over half of patients with Graves' disease
- TSI bind to the TSH receptor and activates the receptor, leading to an increased production and release of thyroid hormone into the blood
- TSI stimulates the thyroid gland to grow
- TRAb binds to the TSH receptor and blocks TSH from binding, resulting in reduced THS receptor function and reduced thyroid hormone production.
If lab tests indicate hyperthyroidism, imaging tests may be used to further determine the cause.
Radioactive iodine thyroid scan-with either I-231 or 99mTc. In this test if the patient's thyroid is scanned, they will swallow radioactive iodine or have an injection of 99mTc. The patient will then wait for the isotope to be taken up by the thyroid gland, and images will be taken to show the amount of isotope taken up by the thyroid.
- This test helps to determine the cause of hyperthyroidism and to assess whether any thyroid lumps or nodules are actively producing thyroid hormone
- Increased uptake of isotope will be seen in a generalized pattern in Graves' disease (See Figure 1 below), and in a localized pattern in toxic nodular goiter (See Figure 2 below)
- Overall decreased uptake of iodine will be seen in subacute thyroiditis (See Figure 3 below)
- "Cold nodules" (swellings in the thyroid gland that do not take up the radioactive isotope on the thyroid scan) may require additional evaluation by fine needle aspiration biopsy to exclude a tumor.
False Positive tests: high total T4 and total T3 levels or suppressed TSH levels
- Estrogen administration or pregnancy can raise levels of TBG (thyroxine-binding globulin), resulting in high total T4 and total T3 levels, but there are normal free T4 and free T3 estimates and normal result on sensitive TSH assay
- Euthyroid hyperthyroxinemia (another condition in which thyroid hormone levels appear to be elevated without an excess function of thyroid hormones) may also be attributable tan inherited condition of other abnormal binding proteins-albumin and prealbumin
- Thyroid hormone resistance states
- Increased serum T4 levels without hyperthyroidism, usually from an inherited condition.
- Administration of corticosteroids, severe illness, pituitary dysfunction
- These conditions may suppress the TSH level in the absence of hyperthyroidism
Medically Reviewed by a Doctor on 6/1/2016
Stephanie L Lee, MD, PhD, FACE
Sonia Ananthakrishnan, MD
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