Physiology · MU Sofia

71. Thyroid hormones: mechanism of action, physiological effects and control of their secretion.

Explains the two thyroid hormones, T3 and T4, and how T4 is converted to T3 and binds nuclear receptors to alter gene transcription. Covers the effects on basal metabolic rate, metabolism, growth, the cardiovascular system and the central nervous system, then the hypothalamic-pituitary control of secretion through TRH and TSH with negative feedback. Ends with recap questions.

Inside: 6 sections · 3 labelled pictures · 15 flashcards · 4 min

Topic 70 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 513 words (4 min of reading), with 15 flashcards.

In this note

  1. Thyroid hormones: mechanism of action, physiological effects and control of their secretion.
  2. Thyroid Hormones
  3. Mechanism of Action
  4. Physiological Effects
  5. Control of Thyroid Hormone Secretion
  6. Recap Questions

Thyroid Hormones

There are two thyroid hormones: triiodothyronine (T3) and thyroxine (T4). T4 is more prevalent, but T3 is more active. They are both based on the amino acid tyrosine and are partially composed of iodine. Therefore, iodine deficiency leads to less T3 & T4 hormone production.

Mechanism of Action

The thyroid hormones act on every cell in the body.

  1. 1. Thyroxine (T4) enters the target cells and is converted into T3 by 5'-iodinase.
  2. 2. T3 enters the nucleus of the cell and binds to the T3-nuclear receptor to form a complex.
  3. 3. The T3-receptor complex causes transcription of many proteins. Some of the transcribed proteins are:
  • Na+/K+-ATPase: increased activity leads to increased O2 consumption
  • Transport proteins
  • β1-adrenergic receptors: binds epinephrine and norepinephrine; in the heart, this causes increased heart rate

Physiological Effects

  • Basal Metabolic Rate (BMR)
  • Increasing O2 consumption is one of the most prominent effects of thyroid hormones.
  • The increased O2 consumption is because of the increased synthesis and activity of Na+/K+-ATPase.
  • Increasing O2 consumption leads to increased BMR and body temperature.
  • The only exception to this increase is the brain, gonads, and spleen.
  • Metabolism
  • Increased O2 consumption requires fuel, so the thyroid hormones cause increased glucose absorption in the gastrointestinal tract.
  • Thyroid hormones also increase the effect of other hormones (e.g., catecholamines, glucagon, and growth hormone) to increase glycogenolysis, gluconeogenesis, lipolysis, and proteolysis.
  • Thyroid hormones increase protein synthesis, but overall the degradation is higher so the effect is mostly catabolic.
  • Growth
  • Thyroid hormones promote bone formation by inducing ossification and fusion of bone plates.
  • Thyroid hormones also induce bone maturation.
  • Cardiovascular
  • The increased O2 consumption caused by thyroid hormones leads to a higher demand for O2, so the heart must increase cardiac output to meet the demand.
  • Thyroid hormones increase the synthesis of β1-adrenergic receptors, making the heart more sensitive to sympathetic stimulation and increasing cardiac output.
  • Central Nervous System (CNS)
  • Thyroid hormones are required for normal development of the CNS.
  • Hypothyroidism is a common problem during infertility.

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