Effect of thyroxine

Rollins-Smith, L.A., Parsons, S.C.V., and Cohen, N., Effects of thyroxine-driven precocious metamorphosis on maturation of adult-type allograft rejection responses in early thyroidectomized frogs, Differentiation, 37,180, 1988. 

Thyroxine is used in hypothyroidism, a condition that may well present in elderly patients. Side-effects of thyroxine usually occur at excessive doses and include gastrointestinal disturbances (nausea, vomiting) as well as cardiac symptoms such as angina pain, arrhythmias, palpitation and tachycardia. Thyroxine should be used with caution in elderly patients as they are more prone to side-effects. A lower initial dose (25-50 pg daily) is recommended for patients who are over 50 years. Dose adjustments should take place at intervals of at least 4 weeks. A pretreatment electrocardiogram is recommended because changes induced by hypothyroidism (that would be present at baseline) may be confused with ischaemia. 

Bimevicius R, Kazanavicius G, Zalinkevicius R, Prange AJ. Effects of thyroxine as compared with triiodothyronine in patients with hypothyroidism. New Engl J Med 1999 340 424-9. 

Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of 02 consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thryroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations Another explanation suggests that the thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation Why ... 

A meta-analysis of studies of the effect of thyroxine in TSH-suppressive doses on measurements of bone mineral density has suggested a small but statistically significant adverse effect on bone mineral density, especially in postmenopausal women (39). 

Gabos, M., Pora, E.A. and Race, L. (1973). Effect of thyroxine (T4), TSH and thiouracil TU (thiourea) treatment on the oxygen consumption of the carp. Studii si Cercetdri de Biologie, Seria Zoologie 25,39-43. 

Siegel NJ, Gaudio KM, Katz LA, et al. 1984. Beneficial effect of thyroxin on recovery from toxic acute renal failure. Kidney Int 25 906-911. 

Larval growth antagonizes effects of thyroxine on metamorphosis. 

Jones, R.W., W.C. Gibson and C.J. Nickolls. Factors influencing mitotic activity and morphogenesis in embryonic development. I. The effects of thyroxine and thiouracil on the development of Brachydanio rerio (zebra fish). Anat. Rec. Ill 93, 1951. 

Mclver B, Rae R Beckett G, et al. Lack of effect of thyroxine in patients with Graves hyperthyroidism who are treated with an antithyroid drug. N Engl 1 Med 1996 334 220-224. 

Adverse effects of thyroxine therapy are due to overdose and are essentially those seen with hyperthyroidism together with the risk of angina, cardiac arrhythmia or cardiac failure. 

Patients with long-standing hypothyroidism, especially elderly ones, are highly sensitive to the stimulatory effects of thyroxine upon cardiac function. Administration of regular doses can cause overstimulation of the heart and cardiac collapse. The answer is (D). 

Hypothyroidism and Major Cerebral Metaboiites Effects of Thyroxine Therapy on /V-Acetyi Aspartate and Choiine Leveis... 

Maternal and fetal hypothyroidism due to iodine deficiency results in significant changes in brain MR spectra. Early thyroxine therapy normalizes the levels of spectral metabolites. The decrease in NAA/Cr ratios, or absolute NAA levels, may be used as a primary indicator of intrauterine hypothyroxinemia-related subtle neuronal damage. In that context, MRS may also demonstrate the therapeutic effect of thyroxine in an objective manner. 

It is apparent, then, that some of the in vitro effects of thyroxine can be explained by an interaction of the hormone with essential metal ions in the medium to date there is no evidence for a direct interaction, either in vitro or in vivo, between thyroxine and a metal-containing enzyme. Further, it must be pointed out that not all inhibitory effects of thyroxine on enzymes can be attributed to the formation of metal-thyroxine complexes. Acetylphosphatase, which is markedly inhibited by thyroxine, is apparently not dependent upon Mg++, Mn++, Cu++, Co++, Fe++, Fe+++, Zn++, or Ca++ for its activation (Harary, 1957). Finally, it should be noted that no in vivo alteration of metal metabolism sufficient to explain the effects of the hormone has yet been demonstrated. 

Faivre, C. Legrand, A. Rabie, Effects of thyroid deficiency and corrective effects of thyroxine cxi microtubules and mitochondria in cerebellar purkinje cell dendrites of... 

Burki, H.J., and Tobias, C.A., 1970, Effect of thyroxine on the cell generation cycle parameters of cultured human cells, Exp. Cell Res.. 60 445. 

Rabie, A., Favre, C., Clavel, M.C., and Legrand, J., 1979a, Sequential effects of thyroxine on the developing cerebellum of rats made hypothyroid by propylthiouracil, Brain Res.. 161 469. 

Weichsel, M., 1974, Effect of thyroxine on DNA synthesis and thymidine kinase activity during cerebellar development, Brain Res., 78 455. 

L.L. Sarlieve, R. Bouchon, C. Koehl and N.M. Neskovic, Cerebroside and sulfatide biosynthesis in the brain of Snell Dwarf mouse effects of thyroxine and growth hormone in the early postnatal period, J. NeurochCTi. 40 1058 (1983). 

Weiss RM, Noback CR, The effects of thyroxine and thiouracil on the time of appearance of ossification centers of rat fetuses. Endocrinology 45 389 (1949)... 

A. Uziel, C, Legrand and A. Rabie, Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat. I. Morphological study, Dev. Brain Res. 19 111-122 (1985). 

For instance, the concentration of cytochrome in muscle increases after thyroxine is administered but decreases in hypothyroid animals. However, this constitutes a nonspecific effect of thyroxine, and other calori-genic hormones have similar effects on the concentration of cytochrome c. 

The chemical structure of the hormone that is active under physiological conditions is still under investigation. Although thyroxine administration undoubtedly corrects the metabolic and clinical alterations that occur after thyroidectomy, and although thyroxine acts on mitochondria in vitro, these observations constitute no proof that thyroxine is the compound that is active in normal physiological conditions. The belief that thyroxine is the active hormone was shattered when 3,5, 3 -triiodothyronine was isolated from blood and thyroid and when it was established that triiodothyronine is more potent than thyroxine on a molecular basis. The discovery of active compounds different from thyroxine has stimulated more research on the physiological effects of thyroxine analogs. 

It is somewhat difficult to evaluate the exact effect of thyroxine on nitrogen metabolism because the metabolism is affected by diet and many other factors including the effect of other hormones, such as those... 

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