Serum TSH and the response to radioiodine treatment of toxic multinodular goitre

A retrospective analysis of data from 73 consecutive patients with toxic multinodular goitre treated with iodine-131 (I-131) during a 2-year period was performed to investigate if serum TSH at the time of I-131 treatment influences the outcome. The dose of I-131 was calculated according to a model compensating for thyroid size estimated by palpation and 24-h I-131 uptake. Serum TSH was determined by a third-generation assay with a functional sensitivity of 0.03 mU/l. A significantly more pronounced response to I-131 treatment was observed in patients with TSH>0.0 mU/l than in patients with TSH=0.0 mU/l (P = 0.0006). This difference resulted in a threefold lower frequency of non-responders and a fivefold higher rate of early hypothyroidism in the group with detectable serum TSH. While the high frequency of hypothyroidism among patients with measurable serum TSH can be explained by destruction of normal thyroid tissue, the high frequency of treatment failure in the group with serum TSH=0.0mU/l suggests that autonomous thyroid tissue may also be sensitized to a deleterious effect of I-131 through stimulation by TSH. We conclude that serum TSH has a significant influence on the outcome of I-131 treatment of toxic multinodular goitre. The results of I-131 treatment may be improved by adjustment of the dose of I-131 according to the serum TSH level, in addition to adjustment for goitre size and 24-h I-131 uptake.