Thyroid hormones are transported in blood in two forms
1-Bound forms >99%: Thyroid hormones are transported in plasma bound to plasma proteins mainly Thyroxine - binding globulin (TBG) ⇒ binds T3 and T4.
Thyroxine Binding Globulin (TBG)
- synthesized by the liver. bound to > 99 % of T3 & T4
- Synthesis of TBG is increased by estrogens ( pregnancy, contraceptives) & is decreased by androgens, glucocorticoids & liver disease.
- In these cases, total T4 & total 3 are changed without changes in the free forms CC In hepatic failure, TBG level decreases → ?? total T3 & T4 . In pregnancy, TBG level increases → ?? total T3 & T4 .
2- Free form <1%: The active form(not bound to plasma proteins) Only the free T3 and T4 can cross the cell membrane & affect intracellular metabolism