The aim of this study was twofold. Firstly to assess the post treatment predictive value of various biochemical and immunological tests for early hypothyroidism after 131I therapy for Graves' disease, and secondly to determine whether or not pretreatment with Carbimazole protects against post treatment hypothyroidism. The early changes observed in serum T3, T4, TSH, thyroid microsomal and thyroglobulin antibody levels were found to be of no predictive value. A sharp rise, around 2 months, in TRAb levels following 131I therapy indicated that hypothyroidism was likely to occur. This rise was thought to reflect a greater degree of thyroid damage. Lower levels of thyroglobulin in patients who had become hypothyroid by 12 months after treatment would support this view. Five weeks Carbimazole pretreatment in this relatively small group of patients did not appear to protect against hypothyroidism.