Rebound increase in serum thyrotropin, anti-'microsomal' antibodies and thyroglobulin after discontinuation of...

We assayed serum thyrotropin (TSH), antimicrosomal antibodies (MSA), antithyroglobulin antibodies and thyroglobulin in seven individuals with subclinical autoimmune hypothyroidism during two 6-month periods with L-thyroxine substitution and placebo, respectively. Serum TSH decreased during L-thyroxine administration, with a rebound increase in serum TSH of about 6 months duration during placebo treatment, and a parallel increase in serum thyroglobulin. In agreement with previous observations by other groups, we found decreased serum concentrations of MSA during L-thyroxine treatment in three individuals. In addition, a slow but significant transient increase in serum MSA was recorded during placebo administration in the four individuals who showed the most pronounced increase in serum TSH (greater than 20 mU l-1). This association between serum levels of MSA and TSH is most probably related to antigen presentation at the surface of the thyrocyte. We conclude that changes in serum TSH concentration should be taken into account in the interpretation of MSA in patients with thyroid disease, whether untreated or treated with L-thyroxine.