QS Study

Peculiarities of Thyroid Hormone Transport

Thyroid hormones are hormones produced by the thyroid gland that have roles in metabolism. Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4).

Transport of the thyroid hormones

A large amount of both T3 and T4 are bound to plasma protein. For T4 this free fraction is normally 0.02-0.04% of the total serum T4 concentration, for T3 about 0.3%. In order to have biological activity, the T4 and T3 must, however, cross the cellular membrane from the serum into the target cells. The plasma proteins that bind to thyroid hormones are –

(a) Albumin,

(b) A prealbumin, formerly called Thyroxin Binding Peralbumin (TBPA), now called trans thyretin.

(c) Thyroxin binding globulin (TBG)

Fig: Thyroid Hormone Transport

Transport of T4

Thyroid hormones are present in the blood either bound or unbound to plasma proteins. Albumin has the largest capacity to bind T4 and TBG has the smallest. Normally 99.98% of the T4 in plasma is bounded the free T4 level as only about 2 ng/dl. Of three proteins, albumin has the largest capacity to bind T4 and TBG has the smallest.

  • Biologist T1 /2: 6-7 days.
  • Vd – 10 L
  • Strongly bound to protein.

Transport of T3

Normally 0.2% (03 µg/d1) of the T3 in plasma is free. The remaining 99.8% is protein bound, 46% to TBG and most of the remainder to albumin.

Transport of RT3 – RT3 binds to TBG.

NB: T3 has a shorter half-life than T4.