Question 3(p.2)

College Answer

This question sought an understanding of the physiological effects of thyroid hormones. The major area of weakness for candidates was a lack of detailed understanding of the physiological actions of thyroid hormones and/or providing an answer that predominately listed clinical manifestations. A good answer would have included the following points

• Stimulation of Carbohydrate Metabolism - all aspects of carbohydrate metabolism, including rapid uptake of glucose by the cells, enhanced glycolysis, enhanced gluconeogenesis, increased rate of absorption from the gastrointestinal tract, and increased insulin secretion
• Stimulation of Fat Metabolism - all aspects of fat metabolism are also enhanced
• lipids are released from fat stores and increased oxidation of free fatty acids by the cells - decreases the concentrations of cholesterol, phospholipids, and triglycerides in the plasma, even though it increases the free fatty acids
• Increased Basal Metabolic Rate – increased CMRO2
• Increased Requirement for Vitamins
• Increased vasodilatation, Cardiac Output, heart rate (not BP), contractility
• Increased Respiration – secondary to increased metabolism
• Increased Gastrointestinal Motility and secretions
• Excitatory Effects on the Central Nervous System, seizures and insomnia
• Effect on the Function of the Muscles – stimulates contractility and metabolism, but too much leads to the muscles become weakened because of excess protein catabolism. Also muscle tremor by increased reactivity of the neuronal synapses in the areas of the spinal cord that control muscle tone.
• Effect on Other Endocrine Glands - . increases the rate of glucose metabolism everywhere in the body and therefore causes a corresponding need for increased insulin and glucagon secretion by the pancreas.
• Also, increase bone formation and, as a consequence, increases the need for parathyroid hormone. Thyroid hormone also increases adrenal glucocorticoid metabolism by the liver.

Discussion

The college model answer is an excellent example of what a model answer should look like (i.e. it should list the points considered important). All you can really do is rearrange it into a more pleasing structure:

• Airway effects
  • Increased airway reactivity resulting in bronchospasm
  • Skeletal muscle myopathy leading to reduced upper airway muscle tone
  • Goitre-related effects (lower airway displacement or compression)
• Respiratory effects
  • Increased respiratory demand (increased CO₂ production because of increased metabolic rate)
  • Increased airway resistance
• Cardiovascular effects
  • Increased contractility
  • Decreased peripheral vascular resistance
  • Thus, increased cardiac output
  • Increased myocardial oxygen consumption
• Neurological effects
  • Mainly excitatory effects (insomnia, anxiety, seizures)
  • Neurodevelopmental effects
  • Increased 
• Renal effects
  • Increased renal blood flow
  • Increased clearance rate of renally cleared substances
  • Increased synthesis of vasoactive mediators
• Hepatic effects
  • Increased rate of protein synthesis
  • Increased hepatic blood flow
  • Increased clearance rate of substances metabolised by the liver
• Gastrointestinal effects
  • Increased gastrointestinal motility
  • Increased appetite
• Metabolic effects
  • Increased carbohydrate metabolism (increased hepatic glucose output, increased gluconeogenesis, increased use of carbohydrate as fuel source)
  • Increased fat metabolism (increased peripheral lipolysis, increased hepatic lipid release, increase in free fatty acids and increased rate of lipid use as metabolic fuel)
  • Increased protein turnover (both hepatic synthesis and peripheral catabolism)
• Thermoregulatory effects
  • Increased shivering and nonshivering thermogenesis
  • Mainly by decreasing the efficiency of mitochondrial fuel metabolism via the uncoupling of electron transport (the major reason for increased carbohydrate and fat metabolism)
• Musculoskeletal effects
  • Increased blood flow to skeletal muscle
  • Increased contractility and oxygen consumption by skeletal muscle
  • Increased muscle protein synthesis (in great excess, increased protein catabolism and myopathy)
  • Increased muscle tone and tremor

Stathatos, Nikolaos. "Thyroid physiology." Medical Clinics 96.2 (2012): 165-173.

Stathatos, Nikolaos. "Anatomy and physiology of the thyroid gland." The thyroid and its diseases. Springer, Cham, 2019. 3-12.

Maenhaut, C., et al. "Ontogeny, anatomy, metabolism and physiology of the thyroid." Endotext [Internet] (2015).

Zhang, Jinsong, and Mitchell A. Lazar. "The mechanism of action of thyroid hormones." Annual review of physiology 62 (2000).

Klein, Irwin, and Kaie Ojamaa. "Thyroid hormone and the cardiovascular system." New England Journal of Medicine 344.7 (2001): 501-509.