Thyroxine-binding proteins

Only small amounts of free T are present in plasma. Most T is bound to the specific carrier, ie, thyroxine-binding protein. T, which is very loosely bound to protein, passes rapidly from blood to cells, and accounts for 30—40% of total thyroid hormone activity (121). Most of the T may be produced by conversion of T at the site of action of the hormone by the selenoenzyme deiodinase (114). That is, T may be a prehormone requiring conversion to T to exert its metaboHc effect (123). 

D24. Docter, R., van Toor, H., Krenning, E. P., de Jong, M., and Hennemann G., Free thyroxine assessed with three assays in sera of patients with nonthyroidal illness and of subjects with abnormal concentrations of thyroxine-binding proteins. Clin. Chem. 39,1668-1674 (1993). 

SchusslerGC. 2000. Tlie thyroxine-binding proteins. Thyroid 10 141-149. 

It would appear that there is a sudden increase in thyroid activity in terms of available or free hormone and an alteration in thyroxine-binding protein which starts probably during surgery and anesthesia and is associated with an increased peripheral utilization of thyroid hormone. Although changes in protein-bound iodine (FBI) and TSH concentrations are not necessarily related to secretion rates, the exact extent of any increase in secretion of thyroid hormone secretion remains uncertain. 

All proteins in the body, whether occurring in the plasma, as membrane-bound proteins, or inside the various compartments of the cell, are degraded or hydrolyzed. The rate of hydrolysis of any specific protein is characteristic of and unique for that protein. Albumin, transferrin, thyroxine-binding protein,and retinol-bind-... 

Transferrin an iron-transport protein in the plasma, has a half-life of about 8 days thyroxine-binding protein, 2 days and retinol-binding protein, 0,4 days-Because of the relatively long half-life of albumin, the serum albumin level is not a particularly sensitive indicator of dietary protein status. Therefore, retinol-binding protein and thyroxine-binding protein have been used to assess malnutrition in poorly nourished populatiorrs. Note that these two proteins occur together as a complex in the bloodstream. 

Cyr, D.G. and J.G. Eales. The effects of short-term 17/3-estradiol treatment on plasma thyroxine-binding proteins in rainbow trout, Salmo gairdneri. J. Exp. Zool. 262 441-446, 1992. 

Until recently it was not always possible to measure unbound thyroxine in serum directly because of the minute quantities involved. Therefore a free thyroxine index calculation was often made, since this value is proportional to the free thyroxine concentration. This index can be arrived at by multiplying a measure of thyroxine in the blood (usually the FBI) by a value derived from the measurement of thyroxine-unoccupied binding sites of thyroxine-binding proteins. This was possible since the uptake of thyroxine or triiodothyronine by red cells or resin is inversely proportional to the number of unoccupied binding sites. As thyroxine-binding sites become increasingly saturated, the free thyroxine levels increase. An example of this method of arriving at a free thyroxine index is the work of Clark and Horn (C5). Calculation of the free thyroxine index is valuable since the FBI determination and the resin uptake of Ts are determinations that are routinely made in many clinical laboratories. 

Ingbar, 8. H., Pre-albumin Thyroxine binding protein of human plasma. Endocrinology 63, 256-259 (1958). 

Schatz, D. L., Serum free thyroxine and thyroxine-binding protein studies in patients with supraventricular tachycardias. J. Clin. Endocrinol. Metah. 27, 165-172 (1967). 

Emerson, C. H., J. H. Cohen, R. A. Young, S. Alex, and S. L. Fang. 1990. Gender-related differences of serum thyroxine-binding proteins in the rat. Acta Endocrinology Copenhagen 123 72-78. 

G22, Cuardn, A., Determination of serum thyroxine by saturation analysis of thyroxine binding proteins. J. Nucl. Med. 10, 532-539 (1969). 

LI. Lamberg, B.-A., Kantero, R.-L., Saarinen, P., and Widholm, O., Endocrine changes before and after the menarche, III. Total thyroxine and free thyroxine index and the binding capacity of thyroxine binding proteins in female adose-scents. Acta Endocrinol. (Copenhagen) 74, 685-694 (1973). 

Takemura, Y., Hocman, G., and Sterling, K., Thernjal stability of serum thyroxine-binding proteins. /. Clin. Endocrinol. Metah. 32, 222-224 (1971). Taurog, A., Porter, J. C., and Thio, D. T., Nature of the T-compounds released into the thyroid veins of rabbits, dogs and cats, before and after TSH administration. Endocrinology 74, 902-913 (1964). 

A.M. Lennon, J. Osty and J. Nunez. Cytosolic thyroxine-binding protein and brain development. Molec. Cell. Endocrinol. 18 201-214... 

R. Ekins, Role of serum thyroxine-binding proteins and maternal thyroid hormones in... 

Ekins, R. P. Hypothesis The roles of serum thyroxine binding proteins and maternal thyroid hormones in foetal development. Lancet (i) 1129-1132, 1985... 

Vitamin A is transported m the plasma as retinol bound to a carrier protein, called retinol-binding protein (RBP), which itself forms a complex with the thyroxine-binding protein, known as transthyretin (TTR). This complex exists in equilibrium with free holo-RBP, which can then interact with a specific cell-surface receptor, thereby inducing the release of its retinol to the target cell. Thus, RBP possesses at least three molecular-recognition properties it binds retinol and it interacts with both TTR and the cell-surface receptor (for reviews, see refs. I and 2). 

Huchzermeier, R. E Spring, T. G. Determination of unsaturated thyroxine binding protein sites using fluorescence polarization techniques. Eur. Pat. Appl. EP 0108400,1984. 

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