A rat model of the 131I-induced changes in thyroid function.

Thyrotoxic patients treated with Iodine-131 (131I) often present with low thyroxine (T4), normal triiodothyronine (T3) and raised thyrotropin (TSH) concentrations in serum. We have developed a rat model of this low T4, raised TSH state. Rats were injected with 50, 150 or 450 mu Ci 131I. A dose of 50 mu Ci 131I caused no significant effect on thyroid function, as assessed by serum parameters whereas both 150 mu Ci and 450 mu Ci 131I caused a significant fall in serum T4 concentration accompanied by a significant rise in TSH concentration. In all groups serum T3 concentration was not significantly altered when compared to controls. The clearance of 131I from the rats showed a single exponential curve (t 1/2 3.38 +/- 0.61 days) over the range of 131I doses used. Differing body weights had no effect on the serum T4 changes induced by 131I.