Reciprocal Relation of Thyroid-Stimulating Hormone and Thyroid-Stimulating Immunoglobulin in a Patient with Endogenous Subclinical Hyperthyroidism

A 72-year-old white woman with an abnormal serum thyroid-stimulating hormone (TSH) concentration was referred to our facility for a comprehensive evaluation. Circulating thyroxine (T4) and free thyroxine (FT4) concentrations were all in the normal range. Triiodothyronine (T3) concentrations were in the low end or slightly below the normal range. TSH was detectable but was below the limits of the normal range. The patient was clinically euthyroid and was receiving medication only for treatment of hypertension. The clinical and laboratory thyroid function status was consistent with a diagnosis of subclinical hyperthyroidism. The physical examination did not reveal thyroid enlargement, nor was there any evidence for the presence of thyroid nodules or ocular changes suggestive of Graves disease. Among thyroid autoantibodies of particular interest was the presence of a moderate thyroid-stimulating immunoglobulin (TSI) level that was stable and consistently present during a 7-month observational period. At 13 months after the initial visit, TSI antibodies were absent and TSH concentration had returned to the normal range. Based on the TSH agonist activity of TSI and the observed reciprocal relation of TSI and TSH, there was no need to suggest pituitary hypersensitivity to thyroid hormone.