Thyroid gland Flashcards
what cells secrete calcitonin?
C (clear) cells / parafollicular C cells
what is the structure of the thyroid gland histologically?
follicular cells surrounding a follicle which is filled with colloid (glycoprotein matrix containing TH)
between follicles is parafollicular C cells
what important nutrient is required for thyroid hormone synthesis?
iodine
what forms the thyroid hormones?
iodide + tyrosine residues
what enzymes catalyses the conversion of tyrosine to TH’s?
thyroid peroxidase (thyroperoxidase)
where is thyroperoxidase located?
located on the apical membrane of the follicular cell
tyrosine + 1 iodide?
MIT = moniodotyrosine (T3)
tyrosine + 2 iodide?
DIT = diiodotyrosine (T4 / thyroxine)
what plasma proteins does T3 and T4 bind to when they enter back into the plasma?
thyroxine binding globulin
what is the movement of TH from the colloid into the plasma under the control of?
TSH release
what accounts for the long half life of thyroxine (T4)
thyroxine binding globulin has a high affinity for T4 compared to T3 which accounts for its slow release into the plasma
what thyroid hormone is more physiologically active and why?
T3
because thyroid hormone receptor has a higher affinity for T3
because T3 is more physiologically active than T4, how is the efficiency of T4 utilised?
T4 is deionised to T3 by deiodinase enzymes
what factors can increase TRH release from the hypothalamus?
cold
pregnancy
exercise
what hormones are inhibitory to TSH release?
glucocorticoids
somatostatin (GHIH)
what is the function of thyroid hormones in the body?
raise metabolic rate and promote thermogenesis
increase hepatic gluconeogenesis
net increase in proteolysis
net increase in lipolyisis
required for foetal brain development
critical for growth (permissive effects of GH)
what can be the causes of hypothyroidism?
iodine deficiency
hashimotos disease
idiopathic - linked to thyroididits
what cna be the cause of hyperthyroidism?
graves disease
thyroid adenoma
what is graves disease?
autoimmune disease which causes hyperthyroidism
autoantibodies which mimic TSH and bind to and activate the thyroid gland
what are the symptoms of hyperthyroidism?
increased HR/ contractile/ CO and cardiac failure
weight loss, heat intolerence
hyperexcitable reflexes, psychological disturbances
muscle weakness
what are the symptoms of hypothyroidism?
weight gain, cold intolerence
brittle nails, thin skin
sow speech & reflexes, fatigue
slow HR / weaker pulse
how does thyroid hormone dysfunction cause goitre?
increased trophic action of TSH on thyroid follicular cells (hypothyroidsim)
over activity of the gland (hyperthyroidism)
hashimotos and graves disease are an example of primary/secondary/tertiary disorders?
primary
How long would it take before the thyroid gland ran out of thyroid hormone if it stopped producing more?
2-3 months
