Diagnostic Use of Radioiodine in Thyroid Disease

The usefulness of radioiodine in the assessment of thyroid function is well established, and in most hospitals it is in routine use. But it does not offer us a universally valid test free of any limitations, and it is useful to supplement it with other tests of thyroid function.20 For the most part, both the special advantages and limitations of radioiodine tests differ from those of the alternative tests, to which they are therefore complementary; where radioiodine is inapplicable or its results unexpected from the clinical picture, recourse to another test will usually clarify the problem. However, it must be admitted that the reliable detection of partial thyroid failure by any test remains difficult. As a consequence opinions differ on the syndrome, incidence and treatment of mild hypothyroidism. For example, a recent controversy about' metabolic insufficiency,' which was said to respond to triiodothyronine but not to thyroxine treatment, has only been settled by ' double-blind' testing of the response to treatment, e.g. Sikkema.YO Some may think that the technically delicate and costly protein-bound iodine test (P.B. 1271) has filled this loophole. However, Lowrey and Starr,38 who for long have been the protagonists of this test and so perform it more accurately than most, found an incidence of 4-9% of subnormal values amongst various sample groups of the normal working population. These people had no symptoms, so that it is likely that these findings reflect the error of the test rather than of the frequency of a real endocrine abnormality. It must be admitted that radioiodine offers no help here. This is even more a blind spot for radioiodine than for the other tests, but it is practically its only one, provided the ingestion of drugs which might affect thyroid function can be excluded. Radioiodine tests can be used to demonstrate or exclude either complete and near complete failure or any increased activity of the thyroid; and for these purposes their routine methods surpass any other test of thyroid function in efficiency, i.e. in simplicity, sensitivity and accuracy. They can also be used to define more precisely the nature of the disorder in certain thyroid diseases. However, as with all tests, clinically unexpected results always call for careful re-examination of the patient, and perhaps rechecking with other tests.