It has been seen that plasma T4 concentrations are correlated positively to the

It has been seen that plasma t4 concentrations are

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It has been seen that plasma T4 concentrations are correlated positively to the body weight of the fetus and newborn. The availability of these hormones in utero regulates fetal growth by acting as a signal of the nutrient and oxygen supply to the fetus. Fetal thyroid hormones are required for both accretion of fetal mass and differentiation of specific cell types. However the placental transfer of thyroid hormones are sufficient enough and compensate for the fetal thyroid deficiency. This is attributed to their role in regulating the somatotropic axis and local tissue expression of the IGFs (insulin like growth factors) which have major role in fetal tissue accretion (57-60). 4g1. Effect on growth : The effect of low thyroid hormones on fetal growth is evidenced by studies on thyroidectomised fetal animals. It is shown that the protein content of fetal tissues such as the heart, lung, and skeletal muscle is reduced by fetal thyroidectomy. The growth restriction is asymmetrical in the sense that greater effects are seen on the weight of soft tissues than on the length of bones. The appendicular skeleton is more adversely affected than the axial skeleton. The effect on bone metabolism is affected by reducing the osteocalcin, a marker of bone deposition rather than altering the calcium homeostasis in the body. 4g2. Effect on fetal metabolism ( oxygen (O 2 ) consumption) – THs increase O 2 consumption by fetal tissues upto 28% and increase umbilical blood flow. This is by inducing oxidative mechanisms by changing expression and
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activity of the electrogenic Na–K ATPase pump or by acting on the mitochondrial electron transport chain (ETC) and oxidative phosphorylation. These hormones are necessary for normal developmental increments in hepatic glycogen and gluconeogenic enzymes. They have an important role in maturational effects of thermogenic capacity of brown adipose tissue as has been described earlier in the text ( see adipose tissue ). 4g3. Effect on fetal tissue maturation : Thyroid hormones also facilitate maturation and differentiation of fetal tissues evidenced by activation of physiological processes essential for survival immediately at birth such as pulmonary gas exchange, adaptations in cardiac function, hepatic glucogenesis and thermogenesis (60, 61). 4g4. Effect on lung maturation – THs tend to increase the expression of pulmonary b- adrenergic receptors and apical Na channels in the fetus thereby facilitationg lung fluid absorption at birth. It is shown that they also facilitate production of surfactant by synergistic action along with cortisol (62) . 4g5. Effect on fetal heart and cardiovascular system – THs promote a switch from proliferation to hypertrophy and differentiation of the cardiac myocytes by playing an important role in the perinatal switch from b- to a- myosin heavy chains in the sacromeres (63).
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