Thyroxine biochemistry

Several studies have failed to confirm the benefits of combined levothyroxine and liothyroninc therapy. Liothyroninc given once a day results in non-physiologi-cal peak serum concentrations of T3. Modified-release triiodothyronine plus thyroxine can normalize serum biochemistry, but it is not known whether this formulation is superior to levothyroxine alone (15). 

While this pattern of biochemistry does not exclude transient relapse of Graves hyperthyroidism (despite the finding of negative TSH receptor antibodies), or a transient thyroiditis, the authors speculated that indinavir (prescribed in this patient together with stavudine and lamivudine) had inhibited the glucuronidation of thyroxine and hence caused a rise in serum thyroid hormone concentrations. 

Lee, C.G. and Ip, Y.K. (1987). Environmental effect on plasma thyroxine (T4), 3,5,3 -triiodo-L-thyronine (T3), prolactin and cyclic adenosine 3 -5 -monophosphate (cAMP) content in the mudskippers Periophthalmus chrysospilos and Boleophthalmus boddaerti. Comparative Biochemistry and Physiology 87A, 1009-1014. 

More detailed discussions of many aspects of the chemistry and biochemistry of thyroxine are available elsewhere (320, 352, 353, 648, 719). 

P8. Purdy, R. H., Woeber, K. A., Holloway, M. T., and Ingbar, S. H., Preparation of crystalline thyroxine-binding pre albumin from human plasma. Biochemistry 4, 1888-1895 (1965). 

Anderson, R. R., D. A. Nixon, and M. A. Akasha. 1988. Total and free thyroxine and tri-iodothyroine in blood serum of mammals. Comparative Biochemistry and Physiology 89A 401-404. 

The isotope dilution technique has been employed for Ihe determination of about thirty elements in a variety of matrix materials, Isotopic dilution procedures have also been most widely used for the determination of compounds of interest in organic chemistry and biochemistry, Thus, methods have been developed for the determination of such diverse substances as vitamin D, vitamin B12, sucrose, insulin, penicillin, various amino acids, corticosterone, various alcohols, and thyroxine. Isotope dilution analysis has experienced less widespread application since the advent of activation methods. Continued use of the procedure can be expected, however, because of the relative simplicity of the equipment required. In addition, isotope dilution is often applicable where activation analysis is not. 

The changes in iodine metabolism are quite typical. Iodine uptake by the thyroid is low, and the levels of protein-bound iodine are markedly reduced in the blood. Except in special cases (see below), the binding properties of TBG and TBPA in blood are not changed. Determination of free thyroxine concentrations in blood of hypothyroids has yielded values lower than 20% of normal (1.5 pg of free thyroxine iodine per 100 ml). If these symptoms are readily explainable on the basis of modem knowledge of thyroid physiology and biochemistry, the causes of the... 

With the exception of C-I bonds in thyroxines, no compounds analogous to alkyl halides are found in animals. However, as discussed earlier, methylation pathways are known in mammalian arsenic biochemistry, and there may be potential sources of CH3 which could react with phosphines. 

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