8.2.5.5 Effect on 5'-monodeiodinase Certain thionamides, in addition to their known inhibitory effect on iodination and coupling of tyrosine moieties to form T, and Ts, also have the ability to inhibit the peripheral conversion of T to T3. With these compounds the pattern of circulating hormone changes that can be seen is reduced circulating T3, increased TSH and reverse T3(rT3) with normal or elevated T4. (As mentioned previously. T4 is considered to be a prohormone and is converted to the more active T3 in peripheral tissues, which include the pituitary.) Hypothyroidism has been observed in man following the administration of amiodarone, where serum TSH levels were doubled(Borowski et Similar changes have been observed in rats dosed with the colour additive Red No. 3(Erythrosine) where lowered T3 values, elevated T4 values, increased rT3 and increased TSH levels suggest a peripheral metabolizing enzyme inhibitory effect for this compound(Capen and Martin, 1989)