Deiodination of T4 to T3 and rT3 by microsomes from normal human thyroid tissue

An enzyme system capable of monodeiodinating T4 to T3 and to rT3 in a microsomal fraction of normal human thyroid tissue is characterized. The pH optimum for 5'-monodeiodination of T4 to T3 was around 7.0 The Km (apparent) for the conversion of T4 to T3 was 5.7 mumoles/l. PTU, ipodate and rT3 inhibited T4 deiodination to T3. The rT3 inhibition was of the partial competitive type. rT3 accumulation could not be detected at pH 7.4 or lower. At higher pH values rT3 could be measured in the incubates; however, the concentration was much lower than that of T3. The pH optimum was 9.0. Added rT3 disappeared rapidly at pH 7.4. The results suggest a high affinity of the 5'-deiodinase in human thyroid tissue for rT3, as has previously been described in the rat, but in contrast to our previous findings in dog thyroid and liver. Using tissue from 3 highly differentiated thyroid medullary carcinomas, deiodination could not be demonstrated, while it was normal using paraneoplastic thyroid tissue from the same patients. This suggests that the deiodinases studied originated from the follicular and not the C-cells of the thyroid.