Thyroid B&B Flashcards
there is sometimes a third lobe of the thyroid called the ______ that sits above the isthmus
pyramidal lobe
what is the blood supply of the thyroid? (2)
- superior thyroid artery - 1st branch of external carotid artery
- inferior thyroid artery - branch of thyrocervical trunk, off the subclavian
as the embryonic thyroid descends into the neck, it initially maintains connection to the tongue called the ______
thyroglossal duct - disappears later in development, remnants in adult as foramen cecum (in tongue) and pyramidal lobe of thyroid
what is a thyroglossal duct cyst
persistent remnant of thyroglossal duct - presents as painless midline neck mass discovered in childhood
classically moves up with swallowing or tongue protrusion
where is the most common location of ectopic thyroid and why does this make sense?
base of tongue - recall embryonic thyroid descends from the mouth (remnant in adult at tongue foramen cecum)
present as a mass in the tongue, detected when there is increased demand for hormones (puberty, pregnancy)
Which cells of the thyroid synthesize thyroid hormone? Where are they found?
Follicular cells: single layer of epithelial cells lining each thyroid follicle
Thyroid follicles are filled with colloid
What needs to happen for iodide in our diet to be added to thyroid hormone? (4 steps)
- taken up by follicular cells.
- Undergo oxidation
- Undergo organification – added to organic/carbon structure (forms MIT or DIT)
- coupling of MIT/DIT into T3/T4 (MIT + DIT = T3, DIT x 2 = T4)
describe how iodine is taken up by follicular cells of the thyroid
via NIS (Na-Iodine symporter)
iodine (- charge) is taken up with sodium (+ charge), but NIS can also take up other negative charged molecules like perchlorate (ClO4-) or pertechnetate (TcO4-) … therefore, these molecules competitively inhibit iodine uptake
what enzyme is required for iodide oxidation and organification in follicular cells of the thyroid
thyroid peroxidase (TPO): oxidizes iodide, also adds iodine to tyrosine (found on thyroglobulin molecules) for organification and catalyzes coupling reactions to create T3/4
which enzyme is responsible for converting T4 (“pro hormone”) to T3 in peripheral tissues?
5’ deiodinase: removes iodine from 5’ position - drug target for hyperthyroidism
what is the mechanism of propylthiouracil (PTU), and what is it used to treat?
hyperthyroid medication
inhibits TPO (thyroid peroxidase) —> decreased T3/4 production from thyroid gland + inhibits 5’-deiodinase —> decreased T4 to T3 conversion in peripheral tissues
which enzyme does methimazole inhibit, and what is it used to treat?
inhibits TPO (thyroid peroxidase) to treat hyperthyroidism —> decreased T3/4 production
how can propranolol be used to treat hyperthyroidism?
propranolol: beta blocker and weak inhibitor of 5’-deiodinase (converts T4 to T3)
great drug in patients with thryotoxicosis (because it’s not super strong)
also blocks catecholamines produced in hyperthyroidism
Wolff-Chaikoff Effect
thyroid protects itself from hyperthyroidism induced by excessive iodide in the diet
high levels of iodide INHIBIT organification of iodide —> less synthesis of MIT/DIT, and thus less T3/4 production
explain how amiodarone can induce hypothyroidism (2 mechanisms)
amIODarone: class III antiarrhythmic used for atrial fibrillation, contains iodine
- can cause hypothyroidism via EXCESS iodine, due to Wolff-Chaikoff Effect - self protective measure, high levels of iodide INHIBIT organification of iodide
- drug mimics T4, and can competitively inhibit 5’-deiodinase in peripheral tissues
