• Previous
• Next

Thyroid hormones - Effects

Thyroid hormones act at the level of the nucleus, mitochondria and perhaps plasma membrane.

• Nuclear action: they penetrate into the cytoplasm of the target cell and then into the nucleus where they act on DNA to stimulate or inhibit the synthesis of mARN and the corresponding proteins, for example enzymes Na+/K+-ATPASE, carbamyl-phosphate-synthase, as well as proteins involved in amino acid and carbohydrate transport.
• Mitochondrial action: they induce the decoupling of oxydative phosphorylations which lead to an increase in oxygen uptake without corresponding synthesis of ATP.
• Cellular membrane effect is possible but not well understood.

The effects of thyroid hormones are very complex and are especially known through the consequences of their deficiency. They act on several levels:

• Growth:
  A deficiency before birth gives a defect of ossification but the child is born with a normal size. A deficiency after the birth causes growth retardation.
• Maturation of the central nervous system, mental development:
  The role of thyroid hormones is particularly important in children before their birth and in the first 45 following days, because their deficiency causes cretinism. The severity of this deficiency is explained by the absence of myelin synthesis. A precocious diagnosis of the deficiency is needed for beginning a substitutive treatment necessary to avoid a severe and irreversible mental deficiency.
  Hypothyroidism appearing after two years of age has less consequences on mental development because the symptoms of hypothyroidism are reversible by substitutive treatment.
• Basal metabolism and thermogenesis:
  Thyroid hormones increase basal metabolism and oxygen uptake by tissues liver, skeletal muscle, heart, kidneys, but not the brain. These effects are attributed, at least partially, to a decoupling of oxydative phosphorylations.
• Heart
  They increase cardiac output and especially heart rate, tachycardia is common in patients with hyperthyroidism.
• Striated muscle:
  Hyperthyroidism induces muscle wasting and hypothyroidism a slowing of the contraction.
• Adipose tissue:
  They increase the sensitivity of adipose cells to the lipolytic effect of various hormones, particularly catecholamines. Plasma cholesterol is increased in patients with hypothyroidism.

• Previous
• Next