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GyyyytdffPathophysiology The hypothalamic-pituitary-thyroid axis governs thyroid hormone secretion (see the image below). The hypothalamic-pituitary-thyroid axis. Levels of The hypothalamic-pituitary-
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Pathophysiology
The hypothalamic-pituitary-thyroid axis governs thyroid hormone secretion (see the image below).
The hypothalamic-pituitary-thyroid axis. Levels of
The hypothalamic-pituitary-thyroid axis. Levels of circulating thyroid hormones are regulated by a complex feedback system involving the hypothalamus and pituitary gland.
Although hypothalamic or pituitary disorders can affect thyroid function, localized disease of the thyroid gland that results in decreased thyroid hormone production is the most common cause of hypothyroidism. Under normal circumstances, the thyroid releases 100-125 nmol of T4 daily and only small amounts of T3. The half-life of T4 is approximately 7-10 days. T4, a prohormone, is converted to T3, the active form of thyroid hormone, in the peripheral tissues by 5’-deiodination.
Early in the disease process, compensatory mechanisms maintain T3 levels. Decreased production of T4 causes an increase in the secretion of TSH by the pituitary gland. TSH stimulates hypertrophy and hyperplasia of the thyroid gland and 5’-deiodinase activity, thereby increasing T3 production.
Deficiency of thyroid hormone has a wide range of effects. Systemic effects are the result of either derangements in metabolic processes or direct effects by myxedematous infiltration (ie, accumulation of glucosaminoglycans in the tissues).
The hypothyroid changes in the heart result in decreased contractility, cardiac enlargement, pericardial effusion, decreased pulse, and decreased cardiac output. A Swedish study, by Mourtzinis et al, found the rates of hypothyroidism and hyperthyroidism in patients with atrial fibrillation to be 5.9% and 2.3%, respectively, compared with 3.7% and 0.8%, respectively, in controls. [7]
In the gastrointestinal (GI) tract, achlorhydria and prolonged intestinal transit time with gastric stasis can occur in hypothyroidism. Delayed puberty, anovulation, menstrual irregularities, and infertility are common. TSH screening should be a routine part of any investigation into menstrual irregularities or infertility.
Decreased thyroid hormone effect can cause increased levels of total cholesterol and low-density lipoprotein (LDL) cholesterol and a possible change in high-density lipoprotein (HDL) cholesterol because of a change in metabolic clearance. In addition, hypothyroidism may result in an increase in insulin resistance.
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Pathophysiology
The hypothalamic-pituitary-thyroid axis governs thyroid hormone secretion (see the image below).
The hypothalamic-pituitary-thyroid axis. Levels of
The hypothalamic-pituitary-thyroid axis. Levels of circulating thyroid hormones are regulated by a complex feedback system involving the hypothalamus and pituitary gland.
Although hypothalamic or pituitary disorders can affect thyroid function, localized disease of the thyroid gland that results in decreased thyroid hormone production is the most common cause of hypothyroidism. Under normal circumstances, the thyroid releases 100-125 nmol of T4 daily and only small amounts of T3. The half-life of T4 is approximately 7-10 days. T4, a prohormone, is converted to T3, the active form of thyroid hormone, in the peripheral tissues by 5’-deiodination.
Early in the disease process, compensatory mechanisms maintain T3 levels. Decreased production of T4 causes an increase in the secretion of TSH by the pituitary gland. TSH stimulates hypertrophy and hyperplasia of the thyroid gland and 5’-deiodinase activity, thereby increasing T3 production.
Deficiency of thyroid hormone has a wide range of effects. Systemic effects are the result of either derangements in metabolic processes or direct effects by myxedematous infiltration (ie, accumulation of glucosaminoglycans in the tissues).
The hypothyroid changes in the heart result in decreased contractility, cardiac enlargement, pericardial effusion, decreased pulse, and decreased cardiac output. A Swedish study, by Mourtzinis et al, found the rates of hypothyroidism and hyperthyroidism in patients with atrial fibrillation to be 5.9% and 2.3%, respectively, compared with 3.7% and 0.8%, respectively, in controls. [7]
In the gastrointestinal (GI) tract, achlorhydria and prolonged intestinal transit time with gastric stasis can occur in hypothyroidism. Delayed puberty, anovulation, menstrual irregularities, and infertility are common. TSH screening should be a routine part of any investigation into menstrual irregularities or infertility.
Decreased thyroid hormone effect can cause increased levels of total cholesterol and low-density lipoprotein (LDL) cholesterol and a possible change in high-density lipoprotein (HDL) cholesterol because of a change in metabolic clearance. In addition, hypothyroidism may result in an increase in insulin resistance.
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