Thyroid and parathyroid gland Flashcards
thyroid gland location
on the anterior and lateral surfaces of the trachea.
just below the larynx
hormone made and secreted by the thyroid gland
thyroid hormone - essential for optimal metabolic activity
calcitonin - hormone involved in calcium homeostasis
thyroid gland structure
composed of small spherical sacs called follicles
each follicle is surrounded by follicular cells (simple cuboidal cells) and is the site of thyroid hormone synthesis
parafollicular cells or C cells lie in clusters between the follicles and make the hormone calcitonin
Thyroid hormone synthesis
follicle cells release protein thyroglobulin (TGB) into the follicle
iodine reacts with tyrosine in the TGB molecules
iodised TGB moves into the follicular cells
thyroid hormone detaches from TGB as needed.
T3 and T4 are produced and travel bound to a carrier protein (thyroid-binding globulin) TBG to target cells
T3 and T4
T3 is the active from of the thyroid hormone and is formed quickly
T4 is the more plentiful form and it released in higher quantity.
target cell activation by thyroid hormone
the thyroid hormone is made in advance and stored until required.
travels bound to carrier proteins.
it detaches from the carrier protein and enters the target cell.
T3 binds to the T3 receptor in the nucleus (receptor is already bound to the specific DNA site)
specific genes are activated to transcribe mRNA
mRNA translation occurs in the cytoplasm and specific proteins are synthesised.
thyroid hormone secretion
stimulus = external and internal CNS input
hypothalamus secretes the thyroid releasing hormone
the anterior pituitary secretes the thyroid stimulating hormone.
the thyroid gland secretes thyroid hormones (T3 and T4)
metabolic effect of the thyroid hormone
increase in basal metabolic rate
by increase in synthesis and activity of the Na+ and K+ pump
other effects of the thyroid hormone
stimulated growth (fetus and early childhood) nervous system = normal alertness and reflexes
what is basal metabolic rate (BMR)
BMR is the body’s rate of energy expenditure under basal conditions = person is awake and physical and mental rest, lying down, no muscle movement, at comfortable temp and fasted
old peoples is smaller and mens is bigger
actions of the thyroid hormone
growth
alertness
metabolism
- increased body heat production (increased oxygen consumption and ato hydrolysis)
- stimulate fat mobilisation and oxidation of fatty acids in many tissues. decreased triglyceride and cholesterol levels breakdown.
- stimulate carbohydrate metabolism, enhance insulin - dependant entry of glucose into cells, increase glycogenesis and glycogenolysis.
- increased proteolysis, predominantly from muscle
Hypersecretion of TH
graves disease high metabolic rate weight loss heat intolerant increased heart rate nervousness hair loss thyroid swelling exopthalamos - swelling behind the eyes
diagnosis of graves disease
thyroid stimulating immunoglobulin (TSI) antibody assay
radioactive iodine scan- iodine is concentrated in the thyroid so huge accumulation when scanned
hyposecretion of TH (in adults)
myxedema low metabolic rate weight gain cold intolerant lethargic slow heart rate
hyposecretion of TH (in children)
cretinism
same as in adults but growth is retarded
brain development is inhibited
caused by lack of iodine in mothers diet
maternal iodine supplement is recommended
what is Goitre
iodine deficiency disorder
thyroid gland is unable to make enough thyroid hormone
when stored TH is depleted, TRH and TSH secretion increase to stimulate the thyroid to make more TH.
without iodine TH cannot be made
net result of goitre
loss of negative feedback control and over stimulation of the thyroid glans
excess TSH stimulates growth of the thyroid glans causes swelling.
Calcium homeostasis
calcium is essential for many physiological functions.
the concentration of extracellular (blood) calcium is tightly controlled.
bone is the body’s major store of calcium
immediate adjustments can be made by rapid exchanges of calcium between bone and blood
normally a balance exists between content build-up and breakdown of bone
hormones involved in calcium homeostasis
Parathyroid hormone (PTH) and calcitonin
Parathyroid gland
secrets parathyroid hormone (PTH)
PTH is essential for life
one the posterior part of the thyroid gland
low blood calcium system
stimulus = low blood calcium concentration
parathyroid gland secretes PTH
the kidneys then increase calcium reabsorption and decrease urinary excretion of calcium
the bone increases bone breakdown an increases the release of calcium into the blood.
this results in an increase of calcium in the blood
calcitonin
parafollicular cells or C cells that lie in clusters between the follicles and make the hormone calcitonin.
Calcitonin secretion pathway
stimulus = increase in blood calcium concentration
the thyroid gland (parafollicular cells) secrete calcitonin
the bone decreases bone breakdown and this decreases the release of calcium into the blood
this also decreases the blood calcium concentration
and it is restored back to normal
calcium regulation: PTH and calcitonin
pth and calcitonin have opposite effects n blood calcium concentration
both act on cone (have receptors on bone cells) to increase (PTH) or decrease (calcitonin) bone breakdown
together they maintain homeostasis but it is not controlled by the hypothalamus
what happens when calcium concentration is too low?
hypocalcemia
increased excitability of the nervous system
leads to muscle tremors and spasms or cramps
begins as plasma ca2+ concentration falls below 2.2mmol/L
under 1.9 paraesthesia develops (tingling sensation in the hands and face, muscle cramps and some can cause the larynx to contract tightly and swell which can cause suffocation.
what causes hypocalcemia?
vitamin D deficiency
diarrhoea
thyroid hormones
under active parathyroid glands, parathyroid gland removal
pregnancy and lactation (calcium goes to the baby and not the mother)
what happens when blood calcium is too high?
hypercalcaemia
nerve and muscle cells are less responsive and excitable
at more than 3mmol/L can lead to depression of the nervous system, emotional disturbances, muscle weakness, sluggish reflexes
once plasma levels are above 3.5 can lead to cardiac arrest - the thresholds aren’t met so action potentials don’t happen and the heart stops beating
