THRYROID GLAND Flashcards
(18 cards)
What are thyrocytes?
Thyrocytes are thyroid cells organized into spheres surrounding a central core of fluid called colloid.
What are follicles in the thyroid?
Follicles are structures formed by thyrocytes surrounding colloid.
What is the major component of colloid?
The major component of colloid is thyroglobulin, a glycoprotein rich in the amino acid tyrosine.
What happens to tyrosyl residues in thyroglobulin?
Some tyrosyl residues will be iodinated, producing the building blocks of thyroid hormone.
How is iodine transported into follicular cells?
Iodine is actively transported into follicular cells by the sodium/iodide (Na+/I–) symporter located on the basement membrane.
What enzyme catalyzes the oxidation of iodide in the thyroid?
The enzyme thyroid peroxidase (TPO) catalyzes the oxidation of iodide.
What thyroid hormones are produced from thyroglobulin?
Monoiodothyronine (MIT) and diiodothyronine (DIT) are produced, which can couple to form triiodothyronine (T3) or thyroxine (T4).
What is the role of thyroid-stimulating hormone (TSH)?
TSH signals the follicular cell to ingest a droplet of colloid by endocytosis.
What happens to colloid droplets inside the follicular cell?
Colloid droplets are digested by intracellular lysosomes into T4, T3, and other products.
What is the metabolic fate of T4?
Approximately 80% of T4 is metabolized into T3 (35%) or reverse T3 (rT3) (45%).
How does T3 compare to T4 in terms of activity?
T3 is three to eight times more metabolically active than T4 and is often considered the biologically active form of thyroid hormone.
What is a prohormone?
A prohormone is a precursor with minimal hormonal effect.
What are the two main forms of iodothyronine 5′-deiodinase?
Type 1 iodothyronine 5′-deiodinase is the most abundant form, primarily in the liver and kidney, while Type 2 is found in the brain and pituitary gland.
What is the function of Type 1 iodothyronine 5′-deiodinase?
Type 1 iodothyronine 5′-deiodinase is the largest contributor to the circulating T3 pool.
How do certain drugs affect Type 1 deiodinase?
Certain drugs, such as propylthiouracil, glucocorticoids, and propranolol, can slow the activity of type 1 deiodinase.
What regulates Type 2 iodothyronine 5′-deiodinase activity?
Its activity is decreased when levels of circulating T4 are high and increased when levels are low.
What is the hypothalamic–pituitary–thyroid axis?
It is a regulatory pathway that controls thyroid hormone activity through thyrotropin-releasing hormone (TRH) and TSH.
