Physiology of the Thyroid Gland Flashcards
Where is the thyroid gland located?
- anterior to the trachea and below the larynx (midline of the neck).
- can feel an increase in size when the gland hypertrophies (normal weight= 15-20 grams).
Does the thyroid gland of a good blood supply?
YES, bc it releases hormones to the blood.
What is one of the largest endocrine organs?
- thyroid gland
What does the thyroid gland secrete?
- triiodothyronine (T3)= 7%
- thyroxine (T4)= 93%
- calcitonin
** Is T3 or T4 more potent?
- T3 (4x more potent)
* however, it’s presence is much lower than T4
Into what is T4 converted in the blood?
- T4 is converted to T3 in the blood
** What stimulates the thyroid gland to secrete T3/T4?
- thyroid stimulating hormone (TSH)
** What gland modulates the metabolic rate?
- thyroid gland
What will you see on histology of the thyroid?
- follicles (contains T3/T4) and colloid/thyroglobulin
*** How much IODINE does the thyroid gland require per week to make thyroxine?
- 1 mg
* iodized salt, fish and seafood are good sources
How is iodine metabolized?
- oral route (PO) absorbed like chloride and excreted by the kidneys.
*** How is T4 synthesized? (see diagram)
- thyroid cells synthesize and secrete thyroglobulin into the follicle (tyrosine amino acids).
- oxidation of iodide ion by PEROXIDASE and HYDROGEN PEROXIDE.
- iodination of tyrosine (organofication) via IODINASE
- mono- di- triiotyrosine progression.
How is iodide added to thyroid hormones?
- Iodide is taken into the thyroid cells (iodide trapping) and secreted into the lumen of the follicle (colloid), where it iodinates thyroid hormones.
How much thyroid hormone is stored in the thyroid gland?
- 2-3 month supply
How is thyroid hormone released into the blood?
- taken from the colloid, cleaved by the lysosomes, and released into the blood.
What happens to thyroxine (T4) in the blood?
- binds to thyroxine-binding globulin (TBG)
* T3 has lower affinity for TBG
What is the mechanism by which T3/T4 work?
- cAMP second messenger system (bind to cell surface)
** What are the effects of thyroid hormones?
- increased PROTEIN SYNTHESIS
- increased PROTEIN CATABOLISM
- increased GROWTH
- increased MENTAL EXCITEMENT
- increase in SYSTEMIC ENDOCRINE ACTIVITY (menses, sweating…)
- increased NUMBER OF MITOCHONDRIA (increases body temp; HEAT)
- increased Na+/K+ ATPase at cellular membranes
- increased FETAL BRAIN DEVELOPMENT
- increased CARBS, FAT, and PROTEIN METABOLISM
- increased NEED for VITAMINS
- increased basal metabolic rate (BMR)
- increased HEART RATE
- increased IONOTROPY (contractility) of the MYOCARDIUM
- increased GI MOTILITY
- increased TREMORS
- increased FREE FATTY ACIDS in PLASMA
- decreased CHOLESTEROL levels in the PLASMA
- decreased PHOSPHOLIPIDS
- decreased TRIGLYCERIDES
- decreased BODY WEIGHT
- decreased sleep
- remember T3 does this better than T4
Is thyroid hormone needed for normal sexual function?
YES
Does hyper or hypothyroidism lead to menstrual defects?
- HYPOthyroidism
What are the effects of TSH?
increased:
- proteolysis of thyroglobulin.
- iodide pump activity.
- increased iodination of tyrosine.
- increased size and activity of thyroid cells.
- modulated by cAMP
** How do we REGULATE/SUPPRESS thyroid hormone production? (TEGRITY)
- thiocyante/perchlorate= decreased iodide trapping, not thyroglobulin synthesis
- PTU (propylthiouracil)= decrases thyroid hormone formation.
- iodide in high concentrations= decreases thyroid activity and size.
What are some diseases of the thyroid?
- HYPERthyroidism: Grave’s disease, thyrotoxicosis…
- HYPOthyroidism:
- toxic goiter= continues stimulation of the thyroid due to hormone synthesis breakdown.
- myxedema= deficient hormone production.
What are some diseases of the thyroid?
- HYPERthyroidism: Grave’s disease, thyrotoxicosis (thyroid storm)
- HYPOthyroidism (treat with hormone replacement).
- toxic goiter= continuous stimulation of the thyroid due to hormone synthesis breakdown.
- myxedema= deficient hormone production.
- cretinism
