Thyroid Hormone Physiology

Overview
  • From a molecular perspective, thyroid hormones act very similarly to steroid hormones in that they bind to Nuclear Receptors inside cells which they then activate (See: Basic Hormone Action). Activated nuclear receptors then go on to initiate the transcription of a very wide variety of genes that change the behavior of that target cell. Because so many cell types are responsive to thyroid hormones, the function and activity of entire tissues and organs are affected by the presence of thyroid hormones. The physiological effects of thyroid hormones, discussed below, are the sum total of these tissue- and organ-level effects.
Physiological Function
  • Overview
    • Thyroid hormones serve diverse physiological functions critical for developmental and maintenance of sufficient metabolism. Many of the sequelae of thyroid hormone deficiency and excess can be inferred from these physiological functions.
  • Metabolism
    • Thyroid hormones enhance whole-body metabolism by modulating the levels a number of key metabolic and catabolic enzymes in multiple tissues. By doing so, thyroid hormones enhance the basal metabolic rate and body temperature. This is likely the most important function of thyroid hormones in adults.
  • Autonomic Nervous System
    • Thyroid Hormones tend to enhance sympathetic nervous system activity in a number of organs but especially the heart thus directly increasing heart rate and cardiac output. These cardiac parameters are also enhanced indirectly by thyroid hormones given the hormones effects on whole-body metabolism which result in enhanced oxygen demand.
  • Normal Linear Growth:
    • Thyroid Hormones cooperate with growth hormone to coordinate normal linear growth to adult stature.
  • CNS
    • Thyroid Hormones are critical for proper brain development in fetuses and neonates. In adults thyroid hormones appear to speed up the brain's activity,
  • GI System
    • Thyroid Hormones enhance GI Motility as a result hypothyroidism can result in constipation while hyperthyroidism yields hyperdefecation (this means increased frequency of well-formed stools and not watery diarrhea).