Iodine is essential for the biosynthesis of the thyroid hormones, thyroxine and triiodothyronine. Fish obtain iodine from
water via branchial pumps and from feed sources (Leloup, 1970). The total uptake of iodine depends on the iodine content of
the feed and water (Gregory and Eales, 1975). Under laboratory conditions, rainbow trout derive 80 percent of their iodide
from water, 19 percent from diet, and less than 1 percent by recycling iodide from thyroid hormone degradation (Hunt and
Eales, 1979).