Thyroid Flashcards
What is the thyroid?
The thyroid is a small butterfly shaped gland in the front of the neck just below the Adam’s apple. Consists of 2 lobes weighting approximately 20 g.
Developed as soon as the 11th week of gestation under stimulation of maternal TSH.
Absolute requirement for intrauterine development.
How are thyroid hormones synthesized?
Thyroid hormone synthesis includes the following steps:
Iodide trapped by the thyroid follicular cells.
Diffusion of iodide to the apex of the cells.
Transport of iodide into the colloid.
Oxidation of inorganic iodide to iodine and incorporation of iodine into tyrosine residues within thyroglobulin.
How are thyroid hormones transported?
Thyroid hormones are transported into cells by a specific carrier-mediated uptake mechanism, that is energy dependent.
T4 is usually converted to T3 by de-ionidases, mainly in liver and kidney, but also in muscles and brain.
T4 may also act as an hormone, but with much lower potency. Consequently T4 can be considered a prohormone (a circulating resevoir of thyroid hormones).
What happens when thyroid hormone receptors bind to thyroid hormone response elements (TRE’s)?
After binding to thyroid hormone, the receptor induces changes in gene expression by stimulating or repressing transcription (the same DNA sequence TRE is found in the promoter region of each target gene recognised by thyroid hormone receptors).
What are the metabolic effects of the thyroid hormone?
Increase the basal rate of oxygen consumption and heat production (cold exposure increased thyroid gland activities).
Increase energy expenditure and metabolic rates (physical exercise, ingestion of an excess of calories, increase T3 production, fasting condition decreases TH production).
Increases glucose and fatty acid oxidation.
Increase cardiac output ensuring sufficient oxygen delivery to the tissues. Heart rate, volume, myocardial contraction are enhanced.
Increase glucose absorption from the gastrointestinal tract and potentiate the stimulatory effect of epinephrine, NE, cortisol, glucagon on gluconeogenesis, on lipolysis, ketogenesis and proteolysis.
Increase cholesterol utilisation.
What are the effects of the thyroid hormone on growth and development?
During development:
Stimulates ossification, linear growth of bone and maturation of the epiphyseal bone centres.
Accelerates growth in part by facilitating GH effects and also stimulating its production.
Crucial during the maturation of the CNS.
Enhances the maturation of chondrocytes (cartilage).
During development, thyroid hormones appear to have their most profound effects on the terminal stages of brain differentiation, including synaptogenesis, growth of dendrites and axons, myelination and neuronal migration.
In adults:
Important in the regeneration of skin and hairs, and in ossification.
Regulation of reproductive function.
Function of skeletal muscles.
