The thyroid of most fish is a diffuse organ located along the floor of the gill chamber, but thyroid tissue can also be found in the spleen, heart, and cranial kidney. Goiter may be difficult to distinguish from thyroid carcinoma, and was mistakenly described as such when initially found in salmonids on iodine-deficient diets. | The thyroid of most fish is a diffuse organ located along the floor of the gill chamber, but thyroid tissue can also be found in the spleen, heart, and cranial kidney. Goiter may be difficult to distinguish from thyroid carcinoma, and was mistakenly described as such when initially found in salmonids on iodine-deficient diets. |