Highly Sensitive Immunoradiometric Assay for TSH and Thyroid Radioiodine Uptake as Predictors of Thyroid Suppression Test

Abstract

Suppression of TSH and thyroid radioiodine uptake by doses of either T4 or T₃ were compared in 33 patients in whom Graves' thyrotoxicosis had been treated with thioamide drugs and the medication was discontinued for at least 4 months. Thyroidal radiodine uptake was suppressed in 19 patients and was not suppressed in the remaining 14 patients. Basal TSH levels before suppression were 2.07μU/ml in the former, significantly exceeding those of the latter (0.91 μU/ml). A TSH level of at least 1.2μU/ml befbre suppression is a good predictor of positive thyroid radioiodine suppression with a predictive value of 76%. A level lower than 0.7μU/ml before suppression is a good predictor of negative thyroid radioiodine uptake suppression with a predictive value of 89%. The determination of TSH levels before the thyroid suppression test was helpful in predicting the result, but there were limitations.
In the thyroid suppression test positive group, circulating T₄ was depressed by doses of T₃. In them, the magnitude of T₄ depression correlated with the levels of thyroid radioiodine uptake before suppression. The levels of TSH correlated neither to changes in T₄ nor to those in thyroid radioiodine uptake. This indicates that the thyroid glands which show high radioiodine uptake are sensitive to TSH and are also sensitive to suppression. The elevated sensitivity to TSH probably warrants the disappearance of abnormal thyroid stimulation more precisely.