According to some recent works. thyroxine causes increased Ca++

owever have challenged this idea. According to some recent works. thyroxine causes increased Ca++ ATPase activity of myosin (which in turn may be due to increase in the bulk of myosin in heart muscle due to thyroxine) increased contractility of myocardium. In short, excess thyroxine excess myosin in the myocardium increased contractility of the heart. (ii) Central nervous system In the neonatal slags, up to the age of approximately 1 year, normal amount of thyroxine is required for the satisfactory development of the neive fibers and their myelination. If sufficient thyronine is not available at this stage. the bram becomes small. The child grows in chronological age but his mental development does not occur. On the other hand, if thymune becomes deficient after the full development of the CNS, some mental signs like mental stowing still develops but can be completely cured by thyroid administration. This is not so in the case of hypolhyroidism in the neonatals If the hypolhyroidism is not corrected within about 1 year of birth, no matter how vigoros is the subsequent treatment, satisfactory brain development will not occur( iii) On skeletal system. ThyroHine is required for the growth and maturation of epiphyseal cartilage so that in the absence of this hormone, linear skeletal growth does not occur. Excess thrioxnine causes osleopoiosis because of calcium drainage from the bone (iv)On reproductive system Lack ofthyromne in adult woman of reproducing age usually causes menorrhagia (= excessive menstrual bleeding) but the explanation is not definitely known HypoIhyioidism may be associated with infertility also. (v)0n digestive system. Excess thyrocxine causes increased motitrty of the gastrc intestinal trad (and thus causing diarrhea) and/or, increased appelite. Lack of thyroxine causes reverse symptoms. (vi)0n blood In hypothyroidism a normoeytic normochromic anemia is seen MECHANISM OF ACTION As stated already (chap i, ssc VI, Mechanism of action. Thyroid hormones), thyroid hormones act somewhat like the steroid hormones. The free (not the TBG bound] T4 enters the target cell (almost all tissues are target cells of T4 ,