CLINICAL MEDICINE

Detailed explanation-1: -Normally, about one-third of T4 is converted to T3 and about one-third to rT3. The remainder of T4 is metabolized by different pathways, in particular glucuronidation and sulfation.

Detailed explanation-2: -In the pituitary, 80–90% of T4 is converted to T3, while only about 30–50% of T4 in the peripheral tissue is converted to active T3. This is due to the inefficiency of D1 and the presence of D3 in all tissues of the body except the pituitary that competes with D1 and converts T4 to reverse T3.

Detailed explanation-3: -It is suggested to start T4 + T3 combination therapy in a L-T4:L-T3 dose ratio between 13:1 and 20:1 by weight [1]. The serum FT3/FT4 ratio (pmol/l to pmol/l) in hypothyroid patients replaced with L-T4 is 0.24 (IQR 0.20–0.28), lower than the value of 0.32 (IQR 0.27–0.37) in euthyroid controls [16].

Detailed explanation-4: -The thyroid produces approximately 90% inactive thyroid hormone, or thyroxine (T4), and 10% active thyroid hormone, or triiodothyronine (T3).