Tests of thyroid function in vivo.

Thyroid uptake tests are essentially kinetic tests which reflect the speed with which certain phases of thyroid hormone biosynthesis are taking place. Under equilibrium conditions the rate at which the thyroid accumulates iodine is closely related to the rate of production of thyroid hormone. The nature of the information provided by thyroid uptake measurements is different therefore from that obtained from the measurement of thyroid hormone levels. The two types of test, the one in vivo, the other in vitro, usually correlate with one another, but in certain circumstances, for instance, in patients treated with an antithyroid drug, this correlation no longer holds. Thyroid function is more likely to be assessed correctly when both types of test are utilized. Thyroid uptake measurements have been used for monitoring thyroid function in patients undergoing treatment with an antithyroid drug (Alexander, Harden, and Shimmins, 1966; Alexander, Harden, Shimmins, McLarty, and McGill, 1967) and at the end of a course of treatment in order to find out whether a remission has taken place (Cassidy, 1965). In both instances the object of the investigation is to determine the underlying functional state of the gland rather than the amount of thyroid hormone being produced. In-vitro tests are inappropriate in these circumstances. Although tests carried out during treatment have proved less helpful in predicting the outcome than was anticipated (Alexander, McLarty, Robertson, Shimmins, Brownlie, Harden, and Patel, 1970; Lowry, Lowe, Hadden, Montgomery, and Weaver, 1971) they do give some indication of the likelihood of remission (Alexander, McLarty, Horton, and Pharmakiotis, 1973; Goolden, Williams, and Thalassinos, 1973). Thyroid stimulation and suppression tests have been carried out using circulating thyroid hormone levels as indices of response (Williams, Ekins, and Ellis, 1969a, and b) but thyroid uptake measurements are generally preferred for this purpose because the changes induced are usually of sufficient magnitude to be readily detected by in-vivo methods. Thyroid uptake measurements are essential for establishing the identity of certain types of thyroid disorder and for studying the mechanism of action of drugs which interfere with thyroid hormone biosynthesis. Thyroid uptake measurements are also utilized in determining the therapeutic dose for patients being treated with 131I for thyrotoxicosis. It is often convenient in an outpatient clinic to have available a test which is completed within half an hour or so. This enables appropriate treatment to be started or a suppression test to be arranged when the patient is first seen. If advantage is taken of recent developments in methodology, the measurement of thyroid uptake is a simple procedure and certainly the quickest way of confirming a diagnosis of thyrotoxicosis.