Blood and thyroid tissue samples were compared among adult, jack, and smolt coho salmon from Oregon and Lake Michigan. Blood samples were analyzed for thyroxine and triiodothyronine, and thyroid tissue was examined histologically. Significant differences in estimates of thyroxine and triiodothyronine levels and histological condition indicated that Lake Michigan jack and adult coho salmon were hypothyroid. The authors theorize that the hypothyroid condition is related to the general iodine deficiency in the Great Lakes Region, contributes to the lower fertility and fecundity of the Great Lakes salmon, and exaggerates pesticide accumulation in the fish.