the link is that the reason for the ailment in the first place is lack of iodine. treat the symptoms and the lack of Iodine is still there, brewing up the next phase of dis-ease: cancer.
if he is worried about prostate cancer then he should consider iodine, boron and selenium in sufficient amounts.
iodine supplementation may decrease or eliminate his need for the synthroid. its just an expensive Iodine supplement, being well over half its weight in Iodine content.