describe the composition of the thyroid
composed of follicles that are made of a central colloid surrounded by flat-cuboidal follicular epithelium cells
what does the colloid contain?
thyroglobulin
what are around the thyroid follicles?
C cells (parafollicular cells)
role of parafollicular C cells
secrete calcitonin to lower serum Ca levels
what is special about the thyroid gland?
it is the only gland that requires substance from the environment to synthesis hormones (iodine)
describe the activation of the thyroid gland (HPA)
- hypothalamus secretes TRH
- stimulates anterior pituitary to secrete TSH/thyrotropin
- TSH acts on the thyroid to cause release of T3 and T4
what releases TSH/thyrotropin
thyrotroph cells
what does TSH do to the thyroid to cause release of T3 and T4?
it binds to the TSH receptor on thyroid epithelial cells which activates G proteins which convert GTP to GDP and produce cAMP which increases production and secretion of T3 and T4
what do T3 and T4 do once they are secreted?
they circulate free and unbound
bind to target cells and form a complex than is translocated to the nucleus which binds to thyroid response elements on target genes and activates transcription
also do negative feedback on the anterior pituitary and hypothalamus
steps in thyroid hormone synthesis (T3/4)
- thyroglobulin synthesis (tyrosine made in the follicular cell)
- uptake and concentration of iodide: iodide ions move to the lumen of the follicular cells that border the colloid
- oxidation 2I- produce I2 which passes through the follicle cell membrane into the colloid
- iodination of thyroglobulin: in the colloid, peroxidase enzyme link iodine to tyrosine amino acids in thryoglobulin to produce a tyrosine attached to I- (MIT) or 2I- (DIT)
- secretino: colloid is enveloped by microvilli to form colloid vesicles that fuse with lyosomes. Enzymes break down lysosomes and release T3 and T4
what serum proteins are thyroid hormones transported on?
TBG
TBPA
albumin
which thyroid hormone is active?
T3 which has a rapid onset and offset
what does metabolic state correlate to the concentration of?
free T3
thyroid hormone effects
- increase basal metabolic rate and glucose uptake
- increase thermogenesis
- increase blood glucose
- increase fatty acid oxidation
- increase protein synthesis
- increase GH
- increase responsiveness to adrenaline and NA
- increase rate and force of cardiac contraction
- increase BR
three types of degradation of thyroid hormones
type 1= liver and kidney
type 2= heart, skeletal muscle, CNS, fat, thyroid and pituitary
type 3= foetal tissue, placenta and brain, except pituitary)
where are the parathyroid glands located?
four glands on the posterior thyroid
what are the parathyroid glands composed of?
chief cells supported by oxyphil cells
what does the parathyroids secrete?
PTH which acts on Ca
