Thyroid Hyperplasia and Tissue Iodine Content in Spawning Rainbow Trout: A Comparative Study of Lake Michigan and California Sea-Run Trout

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Abstract
Rainbow trout taken on their spawning migration in Michigan streams exhibited marked thyroid hyperplasia in most of the sexually ripe or spent fish, in contrast to the relatively quiescent state of the thyroids of immature trout found among the migrating population. Trout with gonads in the process of maturation exhibited varying degrees of thyroid activity. Thyroid hyperplasia in these spawning fish is associated with the low I content of the Great Lakes. Distr. of I in the different tissues of the sexually mature Michigan trout showed the highest concn. of this element in the eggs, 21 [mu]g./100 gm. of tissue compared to 2.7 [mu]g. for the blood serum, 0.74 for muscle, and 1.6 for the testes. When the total I content of the several tissues was detd., the egg mass was found to contain more I than the combined total of all the other tissues, including the thyroid. Analogous observations on the sea-run trout revealed the same pattern of I distr. The actual quantities of I found in the different tissues, however, were 10-20 times greater than those in the Michigan fish.