回复：6 months

The radiactive iodine will be selectively concentrated in the thyroid gland so the radiation effect to the rest of the body will be transient. For I-123 20 mCi of total radiation activity,the body dose in mRad is about less than a single lateral lumbar spine film.

Waiting for 3-6 months will be a good bet because the sperms only live for about 120 days. After that, new sperms will be regenerated (not like females who has about 400 eggs for lifetime.)

Hope that helps.