Thyroid Gland Flashcards
The thyroid gland produces iodothyronines ____ and _____
Tetraiodothyronine (T4) aka thyroxine
Triiodothyronine (T3)
Peripheral conversion of iodothyronines by ____ is key
Deiodinases
The functional unit of the thyroid gland
Thyroid follicle
The thyroid follicle is lined by _____ epithelial cells and the lumen is filled with _____. Follicles also contain parafollicular C cells which secrete _____
Cuboidal; colloid; calcitonin
What is the main secretory product of the thyroid?
T4 (10x more than T3)
Colloid is composed of newly synthesized thyroid hormones attached to ____
Thyroglobulin
Conversion of T4 to T3 occurs in the thyroid and peripherally through the action of ______
Deiodinase
[90% is peripherally converted vs. 10% that is directly secreted]
What are some clinical states that are associated with reduced conversion of T4 to T3?
Fasting
Medical and surgical stress
Catabolic disease
_______ is often associated with elevated levels of type 1 deiodinase, contributing to elevated T3 in the blood
Hyperthyroidism
T4 may be converted to T3 by ____ ring deiodination (activation) by deiodinases type ___ and ___
Outer; 1; 2
What is the primary source of dietary iodine?
Iodized table salt
[also from dairy products and fish]
Synthesis of thyroid hormone involves basal to apical movement of 2 precursors ____ and _____
Thyroglobulin
Iodide
[iodide is taken up from blood; thyroglobulin is assembled from translated amino acids]
______ is the process of binding iodine with thyroglobulin
Organification
What are the important transporters in the basolateral membrane of cells synthesizing thyroid hormone?
Na/K-ATPase
Na/I symporter (NIS, 2:1 ratio)
What are the important transporters in the apical membrane of cells synthesizing thyroid hormone?
Pendrin — Cl/I countertransporter
Peroxidase — oxidizes iodide for combination with thyroglobulin (aka TPO)
After iodination by thyroid peroxidase, thyroglobulin is stored in the lumen of the follicle as colloid.
Within the colloid, thyroglobulin binds to ____, ____, and intermediates ___ and ____
T4; T3; MIT; DIT
When iodide availability is restricted, formation of _____ is favored
T3
Intracellular steps of thyroid hormone synthesis involve _______ of colloid droplet, at which time ______ cleave T4 and T3 from thyroglobulin for release into circulation
Pinocytosis; proteases
[purpose is to rapidly deiodinate MIT and DIT molecules that have been released during proteolysis of thyroglobulin. This iodide is then recycled into T4 and T3 synthesis]
T/F: a deficiency in intrathyroidal deiodinase mimics a dietary iodide deficiency
True
What are some inhibitors of NIS transporter in basolateral membrane?
Perchlorate; thiocynate
What is the primary inhibitor of TPO?
Propylthiouracil (PTU) - inhibits production of final thyroid products
The wolff-chaikoff effect results in inhibition of what step in thyroid hormone synthesis?
Organification (process of binding iodine with TG)
Iodine is stored as iodinated _____ of thyroglobulin
Enough hormone is stored as iodinated TG in the follicular colliod to last 2-3 mos. Each TG molecule contains 30 ____ molecules and a few ___ molecules
Tyrosine
T3; T4
Activity of the thyroid gland can be assessed by radioactive iodine uptake. Compare levels at 24 hrs in normal thyroid, hyperthyroid, and hypothyroid
Normal = 25% at 24 hrs
Hyperthyroid = >60% at 24 hrs
Hypothyroid = <5% at 24 hrs
The majority of thyroid hormone circulates bound to plasma proteins (only 1% is free). What are the main plasma proteins that bind thyroid hormones?
Thyroxin-binding protein (TBG) = 70%
Transthyretin (TTR) = 10-15%
Albumin = 15-20%
The main plasma protein that binds thyroid hormone is TBG. It is synthesized by the ____ and has greater affinity for ____
Liver; T4
Circulating TBG can be indirectly assessed with what test?
T3 resin uptake test