Effect of methyl thiouracil on radioiodine thyroidal retention in rats

Some goitrogens like methyl thiouracil (MTU) because of their thynamide grouping act as antithyroid drugs inhibiting the organification of iodide, but do not alter the iodide transport. Their administration to an intact animal, therefore, might alter the thyroidal iodine kinetics. Here an attempt has been made to study the effect of MTU on thyroidal iodine kinetics in rats as well as to find out whether any difference in kinetics could be detected between different radioiodines, viz., 131I, 125I, and 123I. Cumulated thyroidal activity which is a time integral of the activity has been taken as the parameter to represent the sum effect of thyroidal iodine kinetics over a specific time period of interest. From the in vivo thyroidal activity measurements, carried out over extended periods of time, the cumulated activity was calculated for both MTU treated and normal rats that received 131I, 125I, or 123I at different times before the MTU start. Within a day of the start of the MTU there is a rapid loss of thyroidal iodine. The severity of the loss depended upon the time that elapsed between the start of the MTU schedule and the particular radioiodine administered. The absence of isotopic effect on the uptake as well as on the rate of uptake for the three different radioiodine isotopes studied has been brought out.