Thyroid stimulating antibody: an index of thyroid stimulation in Graves' disease?

Abstract

Abstract. Early (20 min) thyroid radio-iodine uptake (ERU) and thyroid-stimulating antibodies (TSab) were determined in 27 untreated unselected patients with Graves' disease at the time of diagnosis. In 21 subjects the same tests were further performed in parallel during combined carbimazole-L-T3 therapy (mean duration of follow-up: 10.8 ± 5.8 months; mean ± sd). TSab was determined by a cAMP-human thyrocyte culture stimulation assay and expressed in μl-equivalent of a TSab standard/ml (μl-eq/ml). Before treatment, ERU, ranging from 15 to 54% of the injected dose (normal ≤ 8% dose) correlated with serum T3 (r: 0.54; P < 0.01); TSab, ranging from 6 to 85 μl-eq/ml was detected in 21/27 patients. There was a significant correlation between ERU and TSab (Spearman rank test: r: 0.57; P < 0.01). During the first months of treatment, 5 of the 21 patients sequentially studied had undetectable TSab levels throughout the study and in these patients ERU decreased by 57% of its initial value; the remaining 16 subjects were divided into two groups according to ERU changes: in group A (9 patients), initial ERU decreased by 50% or more or the absolute value became less than 20% of the dose and TSab decreased from 10.9 ± 4.8 ISP-μl-eq/ml to 5.3 ± 1.6 μl-eq/ml (P < 0.01); in group B (7 patients), the fall of ERU was less than 50% or the absolute value remained greater than 20% of the dose and TSab values remained unchanged. Furthermore, the values of ERU and TSab serially obtained during treatment varied in parallel in the 9 patients of group A, whereas in group B, TSab and ERU evolutions were discordant. These two patterns of TSab and ERU changes could not be related to any clinical or biological findings. We conclude that 1) ERU is more closely related to thyroid overstimulation than TSab in untreated patients with Graves' disease, but that there is a significant relationship between ERU and TSab values; 2) during antithyroid therapy, the TSab decrement was significantly greater in patients in whom initial ERU decreased by 50% or more or the absolute value became less than 20% of the dose than in patients with persistent high ERU levels; 3) the follow-up of these patients during antithyroid-L-T3 treatment showed two patterns of changes for ERU and TSab, parallel in some patients, discordant in others, suggesting an heterogeneity in the in vivo TSab effect from one patient to another.