Hormones continued Flashcards
Where is the thyroid gland and what does it secrete?
Just below the larynx
• On the anterior and lateral surfaces of the trachea
Hormones made and secreted:
Thyroid hormones - essential for optimal metabolic activity
What is the compisiton of the thyroid gland?
Composed of small spherical sacs called follicles
Each follicle is surrounded by follicular cells (simple cuboidal epithelial cells) and is the site of thyroid hormone synthesis (cross sections show rings of cells)
Clear cells or C cells lie in clusters between the follicles and make the hormone calcitonin
How is thyroid hormone synthesised?
Iodine (in diet - very necessary) travels through blood after absorption from gut where it will move to the thyroid
- Iodine enters follicle cell from blood and travels into follicle cavity
- Follicle cells release protein, thyroglobulin (TGB), into follicle.
- Iodine reacts with tyrosine in the TGB molecules. Iodised TGB moves into the follicular cells (lipid soluble hormone that is stored until its needed - store it as part of the thyroglobulin molecule and cut it off as needed)
- Thyroid hormones detach from TGB as needed
What is the difference + similarites between T3 and T4?
T3 = the active form
T4 = the more plentiful form
- They leave the thyroid through the bloodstream
- They travel bound to a carrier proteins (TBG) to target cell
- Made in advance and stored
How does the thyroid hormone activate the cell?
- Travels bound to a carrier protein
- Detaches from the carrier protein and enters the target cell
- T3 binds to the T3 receptor in the nucleus (receptor is already bound to a specific DNA site)
- Specific genes are activated to transcribe messenger RNA (mRNA)
- mRNA translation occurs in the cytoplasm and specific proteins are synthesised
e. g. sodium-potassium pump (ATPase - maintains ion balance inside/ outside of cell)
Response time: 45 mins - days
What is the thyroid hormone secretion pathway?
Stimulus: external and internal stimuli leads to CNS input to hypothalamus → leads to the hypothalamus secreting thyrotropin releasing hormone (TRH) into the blood
–> Anterior pituitary them secretes thyroid stimulating hormone (TSH) into the blood
→ Gets to the thyroid gland where it will force the cleavage of T4 and T3 off of the TGB molecules and into the bloodstream
What are the effects of thyroid hormone?
main - increasing basal metabolic rate by increase synthesis and activity of the Na+ K+ pump
Other:
- Stimulates growth
- Nerous system: normal alertness and reflexes
What happens in negative feedback with the thyroid hormone?
thyroid hormones feedback and inhibit the release of both hormones.
What is a basal metabolic rate?
BMR is the body’s rate of energy expenditure under basal conditions:
- Person is awake, at physical and mental rest, lying down, no muscle movement, at a comfortable temp and is fasted (12-18h)
- Increase the amount of energy the body expends by doing activities
- Changes throughout lifetime
What are appropriate levels of thyroid hormone needed for?
- Growth
- Alertness
- Metabolism
What are the thyroid hormones effects of metablosim?
- Increases body heat productions (increase oxygen consumption and ATP hydrolysis)
- Stimulates fatty acid oxidation (usage of stored fats) in many tissues
- Increase muscle breakdown
- Stimulates carb metabolism (usage of stored glucose), enhances insulin-dependant entry of glucose into cells, increases gluconeogenesis and glycogenolysis
–> Does not cause a increase in blood glucose (relative balance - being secreted and used)
What is Infantile hypothyroidism?
Low metabolic rate
Cold intolerant (unable to generate enough body heat)
Growth is retarded - brain and general growth
Brain development is inhibited
Caused by
Lack of iodine in mother’s diet (Iodine supplementation is recommended)
Mother needs enough iodine for themselves and for fetus developing
What is grave’s disease?
Hypersecretion of thyroid hormone
High metabolic rate (leads to weight loss) Heat intolerant Increased heart rate Nervousness Hair loss Exophthalmos (protruding eyes) Thyroid swelling (in base of neck)
Cause
Autoimmune disorder
How is calcium concentration regulated?
Dietary requirement - around 1000 mg/day
Goes to
→ Digestive tract
Calcium gets absorbed through digestive tract into the plama (need to maintain Ca+ concentration in plasma of around 8.5 - 11 mg/dL)
→ goes from blood to kidneys (filtered by this)
→ Reabsorption into plasma from the kidneys
→ urinary loss = 650mg/day
→ Some of it doesn’t get absorbed, and is faecal loss (350mg/day)
Absorption occurs through digestive tract
→ Can also get Ca into blood by resorption by osteoclasts from bone
→ If levels are to high can have deposition of Ca+ into bone done by osteoblasts
What is calcium essential for?
Calcium is essential for many physiological functions e.g contraction of muscles and release of neurotransmitters (opening of voltage gated calcium channels which allows Ca to enter the axon terminal and signals vesicle fusion to release a neurotransmitter)
