Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
This form of thyroid hormone is 20 to 50 times more biologically active
T4 is converted in many organs (i.e. liver, kidneys) to the more
bioactive T3 with the removal of an iodine by an enzyme cllaed a deiodinase
T3 is often elevated to a relatively higher level than T4 in severe hyperthyroidism.
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 123I 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
"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
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
Administration of corticosteroids, severe illness,
These conditions may suppress the TSH level in the absence of