s, which has had an increasing iodine supply in recent years, the upper limit of normal is now about 25% of the administered dose. This value is higher in areas of iodine deficiency and endemic goiter. The uptake value at a shorter time interval, for example 6 hours, is as valid a test and may be more useful in the infrequent cases having such a rapid isotope turnover that “uptake” has fallen to normal by 24 hours. If there is reason to suspect that thyroid isotope turnover is rapid, it is wise to do both a 6- and a 24-hour RAIU determination during the initial laboratory study. As noted below, rapid turnover of 131-I can seriously reduce the effectiveness of 131-I therapy. Similar studies can be done with 123-I and also technetium.Because of convenience, and since serum assays of thyroid hormones and TSH are reliable and readily available, the RAIU is now infrequently determined unless 131-I therapy is planned. A drawback of this approach is that cases of transient thyrotoxicosis (described below) may be missed unless the typical low RAIU is recognized. To avoid errors, we recommend that the RAIU test be done in patients who are believed to be thyrotoxic with suppressed TSH, but who do not have typical symptoms and/or signs. This may include patients with brief symptom duration, small goiter, or lacking eye signs, absent family history, or negative antibody test result. Obviously other causes of a low RAIU test need to be considered and excluded. Tests measurinjg suppressibility of RAIU are of some historical interest(13-15)
Isotope scanning of the thyroid has a limited role in the diagnosis of thyrotoxicosis. It is useful in patients in whom the thyroid is difficult to feel or in whom nodules (single or multiple) are present that require evaluation, or rarely to prove the function of ectopic thyroid tissue. Nodules may be incidental, or may be the source of thyrotoxicosis (toxic adenoma), or may contribute to the thyrotoxicosis that also arises from the rest of the gland. Scanning should usually be done with 123-I in this situation, in order to combine it with an RAIU measurement.
Thyroid Ultrasound- US exam of the thyroid is sometimes of value in diagnosis. For example, if a possible nodule is detected on physical exam. It also may confirm hypoechogenicity if the diagnosis is actually Hashimoto’s thyroiditis, or intense vascularity of Graves’ disease if a color Doppler flow exam is done.
Determination of antibody titers provides supporting evidence for Graves’ disease. More than 95% of patients have positive assays for TPO (microsomal antigen), and about 50% have positive anti-thyroglobulin antibody assays. In thyroiditis the prevalence of positive TG antibody assays is higher. Positive assays prove that autoimmunity is present, but they do not prove thyrotoxicosis. However, patients with causes of thyrotoxicosis other than Graves’ disease usually have negative assays. During therapy with antithyroid drugs the titers characteristically go down, and this change persists during remission. Titers tend to become more elevated after RAI treatment.
Antibodies to TSH-Receptor-
Thyrotrophin receptor antibody assays (TRAb) have become readily available, and a positive result strongly supports the diagnosis of Graves’ disease(11.1). Determination of TRAb is not required for the diagnosis, but the implied specificity of a positive test provides security in diagnosis, and for this reason the assay is now widely used. The assay is valuable as another supporting fact in establishing the cause of exophthalmos, in the absence of thyrotoxicosis, and high maternal levels predict fetal or neonatal thyrotoxicosis. Measurement of TRAb (TSH-R binding antibodies), any antibody that binds to the TSH-R, is generally available.Assays for Thyroid Stimulating Antibodies (TSAb,TSI) are less available, but are more specific for the diagnosis. Using current tests, both are positive in about 90% of patients with Graves disease who are thyrotoxic. “Second generation” assays becoming available use monoclonal anti-TSH-R antibodies and biosynthetic TSH-