Physiologic Effects Of Thyroid Hormones

The Natural Thyroid Diet

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Growth and Maturation Skeletal System

One of the most striking effects of thyroid hormones is on bodily growth (see Chapter 44). Although fetal growth appears to be independent of the thyroid, growth of the neonate and attainment of normal adult stature require optimal amounts of thyroid hormone. Because stature or height is determined by the length of the skeleton, we might anticipate an effect of thyroid hormone on growth of bone. However, there is no evidence that T3 acts directly on cartilage or bone cells to signal increased bone formation. Rather, at the level of bone formation, thyroid hormones appear to act permissively or synergistically with growth hormone, insulin-like growth factor (see Chapter 44), and other growth factors that promote bone formation. Thyroid hormones also promote bone growth indirectly by actions on the pituitary gland and hypothalamus. Thyroid hormone is required for normal growth hormone synthesis and secretion.

Central Nervous System

The importance of the thyroid hormones for normal development of the nervous system is well established. Thyroid hormones and their receptors are present early in the development of the fetal brain, well before the fetal thyroid gland becomes functional. T4 and T3 present in the fetal brain at this time probably arise in the mother and readily cross the placenta to the fetus. Some evidence suggests that maternal hypothyroidism may lead to deficiencies in postnatal neural development, but direct effects of thyroid deficiency on the fetal brain have not been established. However, failure of thyroid gland development in babies born to mothers with normal thyroid function have normal brain development if properly treated with thyroid hormones after birth. Maturation of the nervous system during the perinatal period has an absolute dependence on thyroid hormone. During this critical period thyroid hormone must be present for normal development of the brain. In rats made hypothyroid at birth, cerebral and cerebellar

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