Decreased activity of type I 5'-monodeiodinase could lead to which physiologic effects:-: (A) Increased plasma triiodothyronine (T3), (B) Increased plasma reverse T3, (C) Decreased plasma thyroxine T4, (D) Increased TSH
Answer is B. Type I 5'-monodeiodinase is the enzyme responsible for the peripheral conversion of thyroxine (T4) to triiodothyronine (T3), primarily in liver and kidney. This enzyme also participates in the breakdown of reverse T3 (rT3) by converting it to 3,3'-diiodothyro nine. Decreased activity of type I 5'-monodeiodinase will therefore decrease plasma levels of T3 and increase plasma levels of rT3 (by decreasing its breakdown). Type II 5'-monodeiodinase is present in the anterior pituitary and central nervous system. Conversion of T 4 to T 3 within the anterior pituitary by this enzyme is responsible for normal feedback inhibition of thyroid stimulating hormone (TSH) secretion. With decreased type I 5'-monodeiodinase activity, plasma levels ofT4 and TSH usually stay within normal limits because the type II enzyme continues to insure normal feedback inhibition by anterior pituitary T 3.