Testicular damage after radioactive iodine (I-131) therapy for thyroid cancer.

Testicular damage was evaluated in twelve men by determination of sperm counts, serum FSH, LH, and testosterone up to 3 1/2 years after treatment with radioactive iodine (131I) therapy for thyroid cancer. Dose-dependent spermatogenic damage with elevation of serum FSH was evident. Full recovery in one and partial recovery in another patient was documented during 2 1/2 years of follow-up. Serial study in two patients documented a small but significant effect of 50 mCi 131I on serum FSH levels and clinically important effects appear to be restricted to men having multiple doses totalling over 100 mCi.