Table 1.

Some Properties and Metabolic Parameters of the Principal TH-Binding Proteins in Serum

	TBG	TTR	HSA
Molecular weight (K daltons)	54*	55	66.5
Structure	Monomer	Tetramer	Monomer
Carbohydrate content (%)	20	0	0
Number of binding sites for T4 and T3	1	2	4
Association constant, Ka (M−1)
For T4	1 × 1010	2 × 108**	1.5 × 106**
For T3	1 × 109	1 × 106	2 × 105
Concentration in serum
(mean normal, mg/liter)	16	250	40,000
Total T4 binding capacity (mg T4 / liter)	0.2	3	>1000
Relative distribution of T4 and T3 in serum (%)
For T4	75	20	5
For T3	75	<5	20
Half-life (days)	5***	2	15
Degradation rate (mg/day)	15	650	17,000

^*Apparent molecular weight on acrylamide gel electrophoresis 60 K daltons.

^**Value given is for the high affinity binding site only.

^***Longer under the influence of estrogen.