Hellekant Thyroid Flashcards
What hormones decrease the release of TSH?
Somatostatin, dopamine, and high glucocorticoids
Where are receptors for TRH found? What kind of receptors are they?
Anterior pituitary; transmembrane Gq receptors
TSH is similar to what other hormones? Why?
Alpha chain of TSH is same as LH, FSH, placental hormone hCG
What chain is specific to TSH and confers specificity of hormonal action?
Beta chain
What cells have TSH receptor?
Thyroid follicular cells
What type of receptor is the TSH receptor?
Plasmamembrane receptor; GCPR
What dietary requirement is needed to synthesize thyroid hormones?
Iodide
How is iodide brought into the thyroid follicular cell?
Via active transport with Na+
After iodide is brought into the thyroid follicular cell, what happens next?
Iodide is transported to follicular lumen and undergoes oxidation to become iodine
What enzyme is responsible for oxidation of iodide in thyroid hormone synthesis?
Thyroid peroxidase
After oxidation to iodine, what is the next step in thyroid hormone synthesis? What enzyme is responsible for this?
Organification of iodine via thyroid peroxidase
After organification, what is the next step in thyroid hormone synthesis? What enzyme is responsible for this?
Coupling via thyroid peroxidase
What does thyroid peroxidase do?
Oxidation, organification, and coupling reactions
What enzyme is responsible for converting T4 to T3?
5-deiodinase
What thyroid hormone is more active? Has a shorter half life?
T3 is more active and has a shorter half life
What proteins bind thyroid hormones?
Thyroid binding globulin (t3=t4), transthyretin (T4>T3), albumin (T3>T4)
What increases the amount of thyroxine binding globulin?
Pregnancy or OCP use (estrogen)
What decreases thyroxine binding globulin?
Hepatic failure, steroid use
What is the function of deiodinase?
Converts T4 to T3
Where is deiodinase I found?
Prevalent in liver and kidney
What is the MOA of propylthiouracil?
Inhibits both thyroid peroxidase and 5-deiodinase
Where is deiodinase II found?
Brian, pituitary, skeletal and cardiac muscle
Where is deiodinase III found?
Brain, skin, placenta
What is the function of deiodinase III?
Deactivates hormones
What does TSH stimulate in the thyroid gland?
Secretion of mature thyroid hormones, increases thyroid peroxidase synthesis, thyroglobulin transcription, and Na/I transport activity
What are the main functions of thyroid hormones?
Regulation of BMR, increased heat production, increased glucose utilizatin, uptake, and synthesis; increases HR and contractility
What is Cretinism?
Congenital lack of thyroid hormone
What are the symptoms of cretinism?
Profound mental retardation, short stature, delay in motor development, coarse hair, protuberant abdomens
What are the signs of cretinism in newborns?
Resp difficulty, cyanosis, jaundice, horse cry, umbilical hernia, marked retardation of bone maturation
What is the importance of assessing thyroid function at birth?
Early diagnosis can prevent permanent mental retardation; development can proceed normally
What effect does thyroid hormones have on the sympathetic system?
Increases beta-1 receptors, resulting in increased HR and contractility
What are the 4Bs of T3?
Brain maturation, bone growth, beta adrenergic effects, basal metabolic rate
What effect does excess thyroid hormone have on bone?
Promote bone turnover, with net bone loss and hypercalciuria
What effect does excess thyroid hormone have on cholesterol?
Increases in LDL receptor number, accelerating LDL clearance
The thyroid-stimulating immunoglobulin test is used to diagnose what disease?
Graves
What is a difference in release of TSH and T3/T4?
TSH has circadian rhythm, but T3/T4 do not
Why does a goiter develop in iodine deficiency?
TSH stimulates Na/I transporter synthesis, thyroid peroxidase and thyroglobulin synthesis; this buildup can result in a mass
Why does a goiter develop in Graves diseas?
Antibodies stimulate TSH receptor, resulting in lots of T3/T4 production
What is levothyroxine?
T4
What is liothyronine?
T3
What drug can be used as T3 replacement?
Liothyronine
What drug class is used for treatment of hyperthyroidism?
Thioamides
What drugs are in the class thioamides?
Methimazole, carbimazole, propylthiouracil, potassium iodide
What is used for thyroid ablation?
Iodine 131
What radioactive iodine is used for thyroid imaging?
Iodide 123
What decreases the absorption of levothyroxine?
Iron, calcium, AlOH, soy
What drug is more potent, methimazole or PTU?
Methimazole is 10x more potent
What is the MOA of propylthiouracil?
Inhibits thyroid peroxidase and inhibits deiodinase I
What is the MOA of methimazole?
Inhibits thyroid peroxidase
What are the uses for iodine in antithyroid therapy?
Thyroidectomy, to treat thyroid storm, and protection from radioactive iodine
What is iodide 123 used for?
Thyroid scan
What is Iodide 131 used for?
Destroy thyroid
What is a major AE caused by propylthiouracil?
Hepatotoxicity
What thioamide can cause ANCA associated antibodies?
PTU
What thioamide is avoided in in 1st trimester?
Methimazole
What is used to treat hyperthyroidism in 1st trimester of pregnancy?
PTU
What teratogenic effects does methimazole have?
Aplasia cutis
