Relationship between serum TSH and the responsiveness of toxic solitary autonomous thyroid nodules to radioiodine therapy

OBJECTIVE: To investigate if serum TSH at the time of 131I therapy influences the outcome.

DESIGN: A retrospective analysis of data on 39 consecutive patients with toxic solitary autonomous thyroid nodules treated with 131I during a 4 year period.

METHODS: Serum TSH was determined by an ultrasensitive RIA with a functional sensitivity of 0.03 mU/l. The 131I dose was calculated blind to the actual serum TSH according to a model compensating for thyroid size estimated by palpation as well as 24 h 131I uptake.

RESULTS: After a mean follow-up period of 30 months, 34 patients (87% of all patients) were euthyroid, three (8%) had responded insufficiently and required further antithyroid therapy, and two (5%) had developed hypothyroidism. No significant difference in the response pattern between patients with suppressed or detectable serum TSH could be demonstrated. The two patients who developed hypothyroidism both had detectable serum TSH at the time of 131I treatment. No other clinical parameter seemed to influence the outcome.

CONCLUSION: There is no clinically significant effect of circulating TSH on the response of toxic solitary autonomous thyroid nodules to 131I therapy. However, keeping the patients subclinically hyperthyroid when receiving 131I treatment may possibly result in a reduced frequency of hypothyroidism.