Thyroid hormones in vivo

That the type II deiodinase represents a common enzyme for the ORD of T4 and rT3 is supported by their mutual competitive inhibition with corresponding Km and /C( values [72-74,82,83]. T3, which is not a substrate for the type II deiodinase, also does not inhibit the deiodination of T4 and rT3 in vitro. In addition to competitive substrate inhibition, other mechanisms exist for the regulation of type II enzyme activity by thyroid hormone in vivo. Experimental hypothyroidism in rats induces a large increase in type II activity in the CNS [71,82], pituitary [72,83,87] and BAT [73] at least in part by prolongation of the half-life of the enzyme [88]. Treatment of hypothyroid rats with T3 produces a rapid fall in type II deiodinase in CNS and pituitary which appears to be due to an accelerated inactivation of the enzyme [88]. 

The synthesis of the thyroid hormone in vivo has been formulated by Harington (1944) as occurring in the following steps ... 

B. Chemical Structure and Calorigenic Potency op Thyroid Hormones in Vivo... 

Sokoloff, L., Roberts, P.A., Januska, M.M., Kline, J. Mechanisms of stimulation of protein synthesis by thyroid hormones in vivo. Proc. nat. Acad. Sci. (Wash.) 60, 652-659 (1968)... 

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