In addition to thyroid function tests (TSH, free T4 (FT4), free T3 (FT3)), additional tests that can help in the diagnosis include:
- Radioactive iodine uptake. A capsule or “cocktail” containing a measured amount of radioactive iodine is swallowed. This is a perfectly safe procedure. Iodine is part of the thyroid hormones and is therefore taken up by the thyroid gland and more will accumulate in the gland if the gland is overactive, as occurs in Graves’ disease. A probe which measures radioactivity, is placed over the thyroid and the amount of radioactivity detected is compared to the amount that was given. A radioactive substance called Technecium-99 pertechnetate is often used as an alternative to radioactive iodine; it behaves like radioactive iodine but it is more convenient to use and delivers a lower radiation dose to the patient.
- TSH Receptor Antibodies (TRAbs). In most patients with Graves’ disease the diagnosis can be made by the doctor following clinical examination together with the results of thyroid function tests and a thyroid scan. Measurement of TRAbs is therefore not always required. The measurement of TRAbs is useful however when the cause of an overactive thyroid gland is unclear or in pregnancy when hyperthyroidism is suspected. Early in a normal pregnancy human chorionic gonadotrophin (hCG) concentrations are high and some women have a brief period of mild hyperthyroidism due to the action of hCG which has some similar chemical features to TSH that is recognised by some of the mothers. This brief period of mild hyperthyroidism is not harmful to the mother or her unborn baby. However, in pregnant patients with Graves’ disease the TRAbs antibodies may cross the placenta and over-stimulate the thyroid gland of the baby. This situation can lead to miscarriage or poor growth of the baby. If the mother is being treated for hyperthyroidism, anti-thyroid drugs may cross the placenta and cause foetal hyperthyroidism. It is therefore essential to identify those pregnant women who have hyperthyroidism due to Graves’ disease. A high concentration of TRAbs towards the end of pregnancy can indicate thyroid disease (thyrotoxicosis) in the baby. Antibodies may also be present in patients who do not have thyroid disease or who have underactive thyroid glands (hypothyroidism) following treatment of Graves’ disease.
The measurement of TRAbs is therefore helpful a) to identify Graves’ disease in pregnant women with hyperthyroidism, and b) in pregnant women who have a past history of Graves disease.
- Thyroid peroxidase antibody is an autoantibody found in most people with Graves’ disease, as well as in Hashimoto’s thyroiditis