Flashcards in Thyroid Physiology Deck (28):
What hormones does the thyroid secrete?
What is the biologically active thyroid hormone? Why?
How do thyroid hormones travel in the blood?
Bound to plasma proteins
TBG ( thyroxine binding globulin)
TBPA (thyroxine binding prealbumin)
What is the most abundant plasma protein transporter for thyroid hormones?
TBG (70 %)
What is the most abundant thyroid hormone?
What thyroid hormone levels correlate with metabolic state?
Free thyroid hormone levels
What are the cellular components of the thyroid gland?
Follicular cells (make ring like structure around colloid)
Colloid (tyrosine containing, thyroglobulin filled)
Parafollicular C cells (secrete calcitonin)
How are thyroid hormones released from the cell?
Iodine uptake into follicular cells
Move into colloidal cells:
Iodine attached to tyrosine residue from thyroglobulin makes MIT and DIT
Stored in colloid thyroglobulin
Pinocytosis of T3 and T4
T3 and T4 released into bloodstream
What is the thyroid hormone axis?
Hypothalamus: TRH secreted (thyrotrophin releasing hormne)
Pituitary: TSH production increased (thyroid secreting hormone)
Thyroid: TSH binds to follicular cells, T3/4 secreted from colloid
Negative feedback from hormones to hypothalamus and pituitary
What action do thyroid hormones have on lipid metabolism?
increase fatty acid oxidation
mobilises adipose tissue fat
What action do thyroid hormones have on growth?
GHRH requires thyroid hormones for secretion (permissive action)
GH somatomedins (permissive action)
What action do thyroid hormones have on CHO metabolism?
increases blood glucose
increases insulin dependant glucose uptake in cells
increase gluconeogenesis and glyconeogenesis
What action do thyroid hormones have on thermogenesis?
hormones released in response to cold stress
What action do thyroid hormones have on basal metabolic rate?
increases number and size of mitochondria
increased respiratory chain enzymes
increased O2 and ATP hydrolysis
What action do thyroid hormones have on foetal/neonatal brain development?
What action do thyroid hormones have on protein metabolism?
increased protein synthesis
What permissive sympathetic actions do thyroid hormones have?
increases response to adrenaline
increases response to sympathetic neurotransmitters
increases no. sympathetic receptors
How will change in levels of TBG affect levels of thyroid hormones in blood?
Changes total hormone level
Not free hormone level (weren't bound anyway)
What conditions can cause an increase in TBG?
Chronic active hepititis
What conditions can cause a decrease in TBG?
Large dose glucocorticoids
How does a change in temperature affect thyroid hormone levels?
How does stress affect thyroid hormone release?
Inhibits TRH and TSH
How do changes in levels of thyroid hormone throughout the day demonstrate a circadian rhythm?
Highest levels at night
Lower levels in am
How do hyper/hypo thyroidism affect ovulation?
What effect does pregnancy have on thyroid hormones?
thyroxine important for development
increased demand on thyroid
plasma protein binding increases
How does pregnancy effect hypothyroid patients?
thyroid can't meet extra demand
How does hCG effect thyroid function?
Acts like TRH