Biological Function of T4 and T3

Biological Function of T4 and T3

• T4 and T3 act on nearly every cell in the body.
• T3 increases the rate of metabolism and oxygen consumption in all body tissues except brain, gonads, and reticuloendothelial system (RES).
• T3 increases the activity of Na+/K+ pump in all body tissues.
• T3 has an anabolic effect.
• T3, along with GH, regulates long bone growth and neural maturation.
• T3 is essential for proper development and differentiation of all human cells.
• T3 plays a role in human heat generation.

Transport of T4 and T3

• Nearly all thyroid hormones in the circulation are protein-bound, while the remaining small-unbound (free) fraction is responsible for biological activity.
• Thyroid hormones are primarily bound to thyroxine-binding globulin (TBG) and to a lesser extent to thyroxine-binding prealbumin and albumin.
• TBG binds T4 with greater affinity than T3.
• The plasma half-life of T4 is higher than that of T3, resulting in a higher plasma concentration of total T4.
• The free fraction of T3 (0.2% of plasma T3) is higher than that of T4 (0.05% of plasma T4).
• The plasma concentration of free T3 is nearly equal to that of free T4, making T3 the primary contributor to biological activity.

Metabolism of T4 and T3

• T4 and T3 are metabolized in the liver.
• They are conjugated with glucuronate or sulphate to be excreted in urine and, to a lesser extent, in feces.