Hypothyroidism preceding hyperthyroidism in a patient with continuously positive thyroid stimulating antibody.

A 62-year-old woman was initially hypothyroid and then developed hyperthyroidism with continuously positive thyroid-stimulating antibody (TSAb). When she visited our hospital with the complaint of the feel of skipping beats, her serum free T4 level was initially low and thyrotropin (TSH) level was slightly elevated. One month after starting the supplement therapy with l-T4, she developed hyperthyroidism with increased 123I-thyroid uptake. TSH-binding inhibitor immunoglobulin (TBII) was slightly elevated only during the hypothyroid stage. Throughout the whole course, TSAb was continuously positive, while thyroid stimulation-blocking antibody (TSBAb) was not detectable. Primary hypothyroidism with TSAb may suggest the possibility of subsequent development of hyperthyroidism.