Goitrogenic action of methallibure (ICI-33828) has been studied in mice gerbil and hedgehog using thyroid weight and histological structure as an index. Liquefaction and vacuolation of thyroid follicles were most prevelant after methallibure administration. 2. The I131 content of the thyroid gland was significantly higher in the methallibure treated groups than in the controls. This denotes a decrease in the rate of discharge of thyroid hormone. 3. Protein bound radioiodine (Pb I131) was low after methallibure administration. 4. Methallibure administration brings about hypertrophy of pituitary thyrotrophs which is also reflected in the increased basophilic cell percentage in gerbil (control: 15.5 percent; methallibure: 22.8 percent). 5. It is concluded that methallibure acts on thyroid function both by a direct effect on the gland as well as by influencing pituitary thyrotrophic activity in enhancing I131 uptake.
