Comparative effects of thyroidal stimulants and inhibitors of normal and tumorous thyroids in xiphophorin fishes.

Spontaneous thyroid tumors in salmonid fishes, which appear to originate as simple hyperplasias, according to Marine and Lenhart (10, 11) and Marine (9), ultimately become invasive and re place fibrous connective tissue, muscle, and bone. Upon treatment with iodine these tumors will regress even after they have become invasive. It is interesting to compare the tumor in fish with thyroidal growths in mice obtained after a year or more of constant treatment with thiouracil or thiourea. The induced thyroid tumors in mice often metastasize to the lungs; in some in stances they may be successfully transplanted (6, 13). If the goitrogens are withheld in certain strains of treated mice, the thyroid tumors and their metastatic growths apparently regress. Thus the invasive cells of spontaneous thyroid tumors of fishes and experimentally induced thyroid tu mors of mice may be controlled by chemo therapy. This raises a question concerning the properties of thyroid tumors in fishes. To what degree do these atypical growths respond to the regulatory in fluences that affect normal thyroid cell growth and differentiation? This point is of great general in terest because, in the past, the simultaneous pos session of both invasive "carcinomatous" powers