Thyroxine-binding capacity

Eckert B, Blaut M, Holzderber M, Lenz H, Angermaier L, Franken N, et al. Electrochemilumi-nescent immunoassay for thyroxin and thyroxin binding capacity using the random-access analyzer Elecsys. Clin Chem 1995 41 S53. 

Several subjects have also been studied whose serum had an elevated thyroxine-binding capacity (B5, F3, 15, J2). Although high PBI levels were present, patients were euthyroid. Free thyroxine concentration was normal in these patients (15), pool sizes were increased, and absolute thyroxine turnover was normal. 

In hyperthyroidism with the attendant large increase in thyroxine output, the PBI increases. There is an associated increase in free thyroxine concentration (16, Oil, S17) and an increase in the thyroxine turnover rate. Total thyroxine binding capacity is reduced as a result of a combination of factors (II, 16, Oil, R9). The number of available binding sites is decreased by additional thyroxine, TBPA is decreased, and in some patients the binding capacity of TBG is decreased. The converse is true in hypothyroidism (16, Oil, R9). 

F3. Florsheim, W. H., Dowling, J. T., Meister, L., and Bodfish, R. E., Familial elevst-tion of serum thyroxine-binding capacity. J. Clin. Endocrinol. Metab. 22, 735-740 (1962). 

Albumin is a major transport facilitator of hydrophobic compounds which would otherwise disrupt cellular membranes. These compounds include free fatty acids and bilirubin as well as hormones such as cortisol, aldosterone, and thyroxine when these materials have exceeded the capacity of proteins normally associated with them. Albumin also binds ions, including toxic heavy metals and metals such as copper and zinc which are essential for normal physiological functioning but may be toxic in quantities in excess of their binding capacity for their carrier proteins. Binding of protons is the basis for the buffering capacity of albumin. 

The binding capacity of serum proteins is altered by oral contraceptives (195) and leads to alterations in the serum concentrations of various substances, including thyroxine, cortisol, and serum iron (196), and in serum iron binding capacity, which are all increased. 

An indirect measure of the concentration of a specific binding protein can be obtained by adding increasing amounts of thyroxine to serum which has been partitioned by electrophoretic means. Saturation, or the maximal binding capacity is a function of the number and capacity of binding sites available [1/mole (013) in the case of TBPA]. The concentration of binding sites is proportional to the amount of thyroxine necessary to saturate the protein. Such methods have been described (010). Reverse-flow techniques can be used to eliminate trailing albumin from the TBG (E3). 

Estrogens increase the binding capacity of TBG (D6, E4), and this is the reason for the increased FBI s found during the administration of these hormones. Pregnancy is also associated with increased FBI s. Absolute thyroxine turnover is normal in these circumstances (D7, S17), and free thyroxine is normal or slightly decreased. Testosterone has opposite effects (E4). 

Oppenheimer, J. H., Martinez, M., and Bernstein, G., Determination of maximal binding capacity and protein concentration of thyroxine-binding prealbumin in human serum. J. Lab. Clin. Med. 67, 500-509 (1966). 

B16. Brookeman, V. A., and Williams, C. M., Evaluation of the resin strip technique for determining serum Ta binding capacity and serum thyroxine. /. Nucl. Med. 12, 55-60 (1971). 

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). 

Serum proteins can bind large amounts of thyroxine. The amount of hormone bound by a specific protein group depends upon the total amount of protein present in the blood and the affinity of the protein for thyroxine. The total binding capacities of TBG, TBPA,... 

Respiratory or metabolic acidosis results in low binding capacities of thyroxine by all binding proteins because binding capacity of thyroxine in acidosis may explain the rapid turnover of thyroxine observed in pulmonary emphysema and hyperemia. The effect of drugs on thyroxine binding to serum proteins probably results from competition for the binding site. 

Hodgson, S.F., Wahner, H.W. Hereditary increased thyroxine-binding-globulin capacity. Mayo Clin. Proc. 47, 720 (1972)... 

A sustained increase of neutral sterol excretion in patients treated with CPIB has been reported. It is of interest to note that a major hypothesis on the mechanism of action of CPIB relates to its capacity to compete with other acids, especially thyroxine, in binding sites of circulating proteins leading to an increase of thyro-active substances in liver. This mechanism may also well explain the drop in serum FFA induced by CPIB... 

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