Interference of dimethyl alpha-(dimethoxyphosphinyl) p-chlorobenzyl phosphate (SR-202) in thyroid hormone metabolism: evidence of inhibition of monodeiodination.

SR-202 is a non-iodinated potential lipid-altering agent. When administered (100 mg) three times per day for 3 days to six euthyroid subjects it was associated with a 30 +/- 3% (mean +/- S.E.M.) fall in 3,3',5-triiodothyronine (T3) (P less than 0.001), a reciprocal 104 +/- 14% rise in 3,3',5'-tri-iodothyronine (reverse T3, rT3) (P less than 0.01), and a 37 +/- 7% rise in thyroxine (T4) (P less than 0.001). Basal and TRH-stimulated TSH did not change. These results suggested that SR-202 was acting as an inhibitor of the peripheral monodeiodination of T4 to T3. During a second study the same subjects received the same dose of SR-202 for a further 3 days following 15 days of progressive substitutive treatment with L-T4, which they continued to take at 200 micrograms/day until the end of the study. Despite higher levels of thyroid hormones in the substituted subjects, similar results were observed, serum T3 falling by 40 +/- 2% (P less than 0.001), serum rT3 and T4 rising by 168 +/- 24% (P less than 0.01) and 37 +/- 9% (P less than 0.01) respectively. These changes provide compelling evidence that SR-202 is an inhibitor of the peripheral conversion of T4 to T3 that acts on thyroid hormone metabolism without provoking a counter-regulatory pituitary response. It might prove to be a useful tool for the clinical investigation of thyroid function.