A case of Hashimoto's thyroiditis with markedly elevated serum thyroglobulin and evidence of its influence on the measurement of anti-thyroglobulin antibody by highly sensitive assays

We present the case of a 66-year-old woman with Hashimoto's thyroiditis, wh o showed extremely high concentrations of serum thyroglobulin (Tg). Serum T g levels were markedly elevated following a slight elevation of serum thyro tropin (TSH) (22,000 ng/mL and 11.1 mu U/mL, respectively). Although elevat ed concentrations of serum Tg declined concomitant with decrease of serum T SH one month later, Tg concentrations remained high (>948 ng/mL) even at no rmal or suppressed TSH levels. There was no evidence of massive thyroid tis sue damage or thyroid tumor. To our knowledge, there have been no case repo rts of such high concentrations of serum Tg (>2x10(4) ng/mL) in the clinica l course of Hashimoto's thyroiditis. Furthermore, we showed evidence that e xtremely high Tg levels could possibly influence the measurement of anti-Tg autoantibody using highly sensitive radioimmunoassays.