Thyroxine-protein interactions. VII. Effect of thyroxine analogs on the binding of 125-I-thyroxine to highly purified human thyroxine-binding globulin

A Review of the Pharmacokinetics of Levothyroxine for the Treatment of Hypothyroidism

Thyroxine hormone has been recognised since the early part of the nineteenth century and levothyroxine has been available since the mid-nineteenth century as a replacement for deficient thyroid hormones. While levothyroxine remains the staple treatment for hypothyroidism even to this day, its optimal use can be challenging. As is often the case with older drugs, the pharmacokinetics of levothyroxine is often under-appreciated or misunderstood and many factors influence the optimal dosing of levothyroxine. This article will review the pharmacokinetics of levothyroxine in the treatment of hypothyroidism and highlight major concepts that should aid both clinicians and researchers.

Molecular conformation of thyroid hormones: structure and binding interactions of thyroxine

In an accurately determined X-ray diffraction study of the thyroid hormone thyroxine (T4), the two independent conformations in the crystal lattice show significant differences in the outer phenyl ring geometry when compared with that of 3,5,3'-triiodothyronine (T3). The major differences between the T4 and T3 structures are a shortened C4'-O4' bond, contraction of the C3'-C4'-C5' angle and an increase in the C3' and C5' angles of T4. These changes can be correlated with the difference in acidity of the 4'-OH of T4 and T3 and help to explain binding affinity differences among thyroactive compounds. The hydrogen bond directionality observed in T4 and other thyroid structures offers an insight into the molecular details of the hormone-receptor site. The conformation of one T4 molecule is cisoid, that of the other transoid, the first such instance of different overall conformations to be found in the same crystal lattice. One T4 molecule has the side chain nearly coplanar with the inner ring, an unusual conformation among thyroid structures.

Thyroxine-protein interactions. Binding constants for interaction of thyroxine analogues with the thyroxine binding site on human thyroxine-binding globulin

The binding constants for interaction of various thryoxine analogues with the thyroxine binding site on human thyroxine-binding globulin have been determined. Equilibrium dialysis, at pH 7.4 and 37 degrees C, was used to measure the competitive effects of different iodothyronine compounds on the binding of 125I-labeled thyroxine to highly purified thyroxine-binding globulin. Relative to L-thyroxine, K = 6 . 10(9) M-1, the association constants of some important analogues were D-thyroxine, 1.04 . 10(9) M-1, 3,5-diiodo-3'-isopropyl-L-thyronine, 4.9 . 10(8) M-1; L-triiodothyronine, 3.3 . 10(8) M-1, 3,3',5'-DL-triiodothyronine (reverse triiodothyronine), 3.1. 10(8) M-1; tetraiodothyropropionic acid, 2.7 . 10(8) M-1; tetraiodothyroacetic acid, 2.6 . 10(8) M-1; 3', 5'- diiodo-DL-thyronine, 8.3 . 10(7) M-1; and 3,5-diiodo-DL-thyronine, 7.1 . 10(7) M-1. Calculation of the deltaG0 values for binding of the analogues indicates that a major contribution to the free energy favoring binding is made by the alanine side chain of thyroxine. A change in configuration of the alpha-amino group from the L to D form causes an unfavorable change of 1 kcal/mol in the free energy of binding. Removal of the alpha-amino group as in tetraiodothyropropionic acid causes an unfavorable change of 1.9 kcal/mol in the free energy of binding. With regard to ring substituents, the results indicate that the two inner 3,5-iodines make about the same contribution to binding as the two outer 3', 5'-iodines.

A new theoretical description of the binding of thyroid hormones by serum proteins

A theoretical model is proposed which describes the binding of thyroid hormones to serum proteins in terms of easily determined parameters, Not only free hormone concentration but also the distribution of hormone among various binding sites may be computed. The mathematical approach is capable of dealing with models of differing complexity of binding from simple 'one binding site per protein molecule' systems to those involving 'negative co-operativity'. The approach gives predictions of thyroid function parameters which are in good agreement with those observed in practice.

Triiodothyronine radioimmunoassay

Highly specific antisera to triiodothyronine (T(3)) were prepared by immunization of rabbits with T(3)-bovine serum albumin conjugates. Antisera with T(3) binding capacity of up to 600 ng/ml were obtained. The ability of various thyronine derivatives to inhibit the binding of T(3-) (125)I to anti-T(3) serum was found to vary considerably. l-T(3), d-T(3) and several triiodoanalogues were potent inhibitors of the reaction. Little inhibition of T(3-) (125)I binding was produced by l-thyroxine (T(4)) or other tetraiodo- analogues, thyronine or iodotyrosines. Chromatography of several T(4) preparations indicated that most of their very slight activity could be ascribed to contamination with T(3). Successful assay of T(3) in serum was accomplished by the addition of diphenylhydantoin to the assay system. Under these circumstances, recovery of T(3) added to serum was excellent, and addition of T(4) was without significant effect. Serum T(3) concentrations in normal subjects averaged 145 +/-25 ng/100 ml (sd). Increased concentrations (429 +/-146 ng/100 ml) were observed in hyperthyroid patients whereas those with hypothyroidism had serum T(3) levels of 99 +/-24 ng/100 ml. Elevated T(3) concentrations were found also in hypothyroid patients receiving 25 mug or more of T(3) daily and in those receiving 300 mug of T(4) daily. Serial measurements of T(3) concentrations in subjects after oral T(3) administration revealed peak T(3) concentrations 2-4 hr after T(3) administration. Intramuscular administration of thyrotropin (TSH) resulted in earlier and more pronounced increases in serum T(3) than in serum T(4) concentrations. Triiodothyronine (T(3))(1) was recognized to be a biologically active secretory product of the thyroid gland over a decade ago (1). Recent studies have indicated that it is formed extrathyroidally as well (2, 3). Nevertheless, relatively little information concerning the role of T(3) secretion in different thyroid disorders has been accumulated until very recently. Methods for the measurement of T(3) which require its extraction from plasma, and often its separation from thyroxine as well, have been described by several investigators (4-11). These methods have proven useful, but they are relatively complicated, the number of samples that can be assayed is limited, and they may be affected by in vitro deiodination of thyroxine. More recently the radioimmunoassay technique has been applied to the measurement of T(3). Several preliminary reports have appeared describing the preparation of antibody to triiodothyronine by immunization of animals with T(3)-protein conjugates and its use for the measurement of T(3) in serum (12-15). The present report describes the development of a radioimmunoassay for the measurement of T(3), studies of the specificity of the anti-T(3) serum, and some initial studies which indicate that the method is applicable to the measurement of T(3) in unextracted serum.