Thyroid Axis Flashcards
(33 cards)
Control of t4 secretion
T4 decreases the sensitivity of thyrotropes to TRH, decreasing TSH secretion
Thyroid hormone synthesis
Thyrocytes uptake iodine and synthesize thyroglobulin, thyroglobulin is exocytosed to the lumen and iodinated
Thyroid hormone release
Iodinated thyroglobulin is endocytosed by thyrocytes and then proteolyzed and released
Thyroid peroxidase
Iodinates tyrosine residues and then condenses residues to form thyroid hormones
Sodium iodide symporter
Transports sodium and iodide on basolateral surface into thyrocyte
Condensation of residues
MITs and DITs combining to form T3 or T4
rT3
Form of T3 with the wrong iodine removed so it has no biological activity
TSH receptors
G protein linked that lead to cAMP and IP3/DAG production to regulate almost all aspects of thyroid hormone synthesis and release
TRH release pattern
Pulsatile but also follows circadian rhythms and youth
Wolff-Chaikoff effect
Excess of iodide consumption inhibited activity of iodine trapping to prevent hyperthyroidism
Secreted ratio of thyroid hormones
4:1
Ratio of thyroid hormone in plasma
20:1
Transport of thyroid hormones
99% bound to thyroxin-binding globulin but only the free parts are biologically active
T3 receptor
In the nucleus, regulate expression of genes
T4 receptor
May also bind to cell membrane receptors and activate intracellular events
Effect of activated thyroid receptors
Enhanced or suppressed gene expression
Isoforms of thyroid receptors
Alpha 1 and 2, beta 1 and 2; cat acts as homodimers and heterodimers)
T4 half life is ____________, T3 half life is ___________
6-7 days, 1 day
Where T4 is converted to T3
In the liver, kidney, brain, and muscle
How thyroid hormones are removed
Conjugated in liver, excreted in bile and kidney
Physiologic actions of thyroid hormone
Involved in synthesis of other hormones, enhance effects of GH, regulate glucose consumption and calorie if hormones, necessary for normal cardiac function
CNS regulation by T3
Regulates dendritic and axonal growth, myelin formation and synapse formation, r regulate neuronal migration
Other effects of thyroid hormone
Hematopoiesis, normal gonadal development, T3 increases K/Na pump activity, number and activity of mitochondria, and ATPase activity
Gestational hypothyroidism
Severely stunted physically and impaired mental development, causing mental retardation, swelling of skin, loss of water and hair, bone thinning, delayed puberty, infertility
