Thyroid Phys- Trachte Flashcards
Describe the hypothalamic pituitary thyroid axis
TRH is secreted by hypothalamus
Initiates TSH secretion by anterior pituitary which goes to thymus
Initiates production of T3 and T4 thyroid hormones
T3 and T4 have systemic metabolic effects
What other regulators DECREASE TRH/TSH release?
Somatostatin
Dopamine
High glucocorticoids
What cells secrete TSH?
Thyrotropes
What type of hormone is TRH? Where is its receptor on thyrotropes of the anterior pituitary?
Protein hormone
Receptor is in the plasma membrane
What type of receptor does TRH bind?
G-protein linked (Gq) because it is couple to PLC and IP3
What type of receptor does TSH bind on the thyroid follicular cells?
Gs coupled receptor
What are the 3 thyroid hormone carrier proteins?
Thyroxine binding globulin (TBG)
Transthyretin
Albumin
Which is more active T3 or T4?
T3!
What would one convert T4 to in order to make it inactive for disposal?
rT3 (basically removing the wrong iodine)
How is iodine transported into the follicular cells?
Linked with Na
Where are the thyroid hormone receptors located on the target cell?
INTRACELLULARLY!
Thyroid hormone MUST cross plasma membrane
Describe the steps following TH binding to it’s intracellular receptor…
The receptor either combines with and THR (homodimer) or retinoid X receptor (heterodimer)
Binds a thyroid response element on DNA
Initiates gene transcription
What are the 4 B’s of T3?
Brain maturation
Bone Growth
B-adrenergic effects
Basal metabolic rate increase
What is cretinism?
Congenital lack of thyroid hormone in infants: profound mental retardation Short stature Delay in motor development Coarse hair Protuberant abdomens
How do most people get enough iodine in their diet?
Iodized salt!
What is hashimoto’s?
autoimmune disease
anti-TSH antibodies??
Causes hypothyroidism
Why is there routine screening for TSH or T4 in the developed world?
Early diagnosis of hypothyroidism can prevent permanent mental retardation
If correction is delayed, physical development can be fixed, but mental delay cannot
Cold intolerance is associated with…..
HYPOthyroidism
Heat intolerance is associated with….
HYPERthyroidism
What would happens to TRH, TSH, and TH in Hashimoto?
Hashimoto is antibodies to protein in thyroid gland - so low TH produced (no negative feedback)
High TSH
High TRH
What happens to TRH, TSH, and TH is pituitary deficiency?
Pituitary deficiency would mean low TSH produced
Thus TH would be low
TRH would be high (no negative feedback)
What happens to TRH, TSH, and TH if there is TH resistance (like a faulty TH receptor)
TH high
TSH high
TRH high
(it is the EFFECTS of TH binding to the receptor that actually induce the negative feedback)
What is grave’s disease?
Production of thyroid stimulating immunoglobulins (TSI)
The antibodies MIMIC TSH
What happens to TH, TSH, TRH in grave’s disease?
Antibodies mimic TSH - so there would be high TH
Low TRH Low TSH (because of negative feedback)
What is a goiter? when do you see it?
Goiter = enlarged thyroid gland
Can be seen in hypo or hyper thyroidism
What would elevated TSH indicate?
Hypothyroidism
What would low TSH levels indicate?
Hyperthyroidism
How do you interpret the radioactive iodine uptake test?
Measures the amount of iodine the thyroid collects from the blood stream.
Low levels of iodine uptake might be a sign of hypothyroidism
High levels would support Grave’s diagnosis
Levathyroxine
T4
Liothyronine
T3
How do thioamides work?
preventing iodide organization and thyroid peroxidase reactions to iodinate tyrosines
Treats hyperthyroidism
(methimazole, propylthiouracil, potassium iodide)
Propylthiouracil
Blocks thyroide peroxidase, inhibiting oxidation of iodine and the conjugation reactions to make T3 & T4
Inhibition of thyroid hormone synthesis
Also blocks 5’-deiodinase so it decreases peripheral conversion of T4 to T3
Methimazole
Blocks thyroide peroxidase, inhibiting oxidation of iodine and the conjugation reactions to make T3 & T4
Inhibition of thyroid hormone synthesis
When would you use I(131)?
Radioactive iodine used from thyroid ablation
When would you use I(123)?
Radioactive iodine used for thyroid imaging
Which drug do you use for hyperthyroidism in pregnancy?
Propylthiouracil
Why does pregnancy require an increased dose of Levothyroxine by 30-50%?
Pregnancy increases serum carrier proteins (and T4 is only active when it is FREE!)
Why do we usually treat with levothyroxine over liothyronine for hypothyroidism?
Levothyroxine (T4) has a longer half life!!
What would you use to treat graves disease?
Thioamides: Propylthiouraci and methimazole
