THYROID GLAND Flashcards
Location and morphology of thyroid gland
The thyroid gland is a butterfly-shaped endocrine organ located in the lower neck, in front of the trachea, just below the larynx.
Function of the thyroid gland
It plays a crucial role in regulating metabolism through the secretion of thyroid hormones (T3 and T4) and calcitonin.
• Ring-shaped structure
Follicle / Follicular cells
• Basic Unit: Spherical structures filled with colloid (a gel-like substance).
• Lined by follicular cells (simple cuboidal or columnar epithelium).
Thyroid Follicles
Thyroid Follicles FUNCTION
Synthesize and secrete thyroid hormones (T3 and T4).
Colloid contains thyroglobulin, a precursor to these hormones.
• Colloid (aka.____ or _____)
- hormones
- thyroglobulin
- glycoproteins
Cavity or Lumen
Colloid
• It stores_____, a precursor protein needed to make thyroid hormones.
• The colloid appears_____ under the microscope when stained (e.g., with H&E stain).
thyroglobulin
pink
Colloid
• It stores_____, a precursor protein needed to make thyroid hormones.
• The colloid appears_____ under the microscope when stained (e.g., with H&E stain).
thyroglobulin
pink
Thyroglobulin
• Thyroglobulin is a large glycoprotein synthesized by_____ cells.
• It is stored in the____ and serves as a template for making_____
• _____is added to thyroglobulin in the colloid to produce these hormones.
follicular
colloid; T3 and T4.
Iodine
Colloid (aka. Cavity)
Synthesizes (3)
- hormones
- thyroglobulin
- glycoproteins
Follicle
• cavity surrounded by simple cuboidal epithelium called…
Follicular Cells
Follicular Cells
•_______ near the capillary blood supply
•_______
- adjacent to the colloid
- synthesis and storage of thyroid hormones
- iodination, exocytosis, and the initial phase of hormone secretion occur
Bases
Apices
Parafollicular Cells
• aka
C cells
Parafollicular cells produces what?
Calcitonin
Role of Calcitonin
• Target:
• Purpose:
Bones, kidneys, and intestines.
To lower blood calcium levels when they are too high.
Roles of calcitonin
• : Prevents osteoclasts (bone cells that break down bone tissue) from releasing calcium into the bloodstream.
• : Increases calcium excretion in the kidneys.
Inhibits Bone Resorption
Promotes Calcium Excretion
Thyroid Hormone
Synthesis
•_____
- major component of thyroid hormone
iodine
Sources of Iodine
• dietary intake
• deiodination of organic iodine-containing moieties within the gland
glycoprotein synthesized by the rough endoplasmic reticulum in the basal and perinuclear regions of the follicular cell
TGB or Tg
• Thyroglobulin (TGB or Tg)
- modified by the______, packaged into_____
• vesicles undergo_____, releasing Tg into the lumen
Golgi complex
vesicles
exocytosis
Thyroid Hormone Synthesis
1-7
- Uptake of lodide
- Oxidation of lodide
- lodination of the tyrosyl residues on Tg
- Coupling
- Proteolysis of Tg
- Deionidation of iodotyrosines
- Deionidation of T, to T3
- Uptake of Iodide
• What happens: Iodide ions (I⁻) are actively transported into the_____ from the blood via a protein called the_______.
• Why: Iodide is the raw material needed to make thyroid hormones.
follicular cells
sodium-iodide symporter (NIS)
- Oxidation of Iodide
• What happens: Iodide ions (I⁻) are oxidized into iodine (I₂) by the enzyme _________in the______
• Why: Iodine is the active form needed for hormone synthesis.
thyroid peroxidase (TPO)
follicular lumen (the colloid).
- Iodination of Tyrosyl Residues on Thyroglobulin (Tg)
• What happens: The enzyme TPO attaches iodine to specific tyrosine residues on thyroglobulin (Tg), a glycoprotein produced by follicular cells.
• Why: This step forms______ and _______ the building blocks of T3 and T4.
monoiodotyrosine (MIT) and diiodotyrosine (DIT),
- Coupling
• What happens: Two iodinated tyrosines (MIT and/or DIT) are coupled together by TPO to form:
• Why: T3 and T4 are the active thyroid hormones.
• T3 (triiodothyronine): MIT + DIT
• T4 (thyroxine): DIT + DIT
- Proteolysis of Thyroglobulin
• What happens: Thyroglobulin, now containing T3 and T4, is endocytosed back into the follicular cells._______ break it down to release T3 and T4 into the bloodstream.
• Why: This step liberates the hormones for secretion.
Lysosomal enzymes
- Deiodination of Iodotyrosines
• What happens: Any leftover MIT and DIT that weren’t used are deiodinated by the enzyme_________, recycling iodine and tyrosine for future hormone synthesis.
• Why: To conserve iodine and prevent waste
iodotyrosine deiodinase
- Conversion of T4 to T3
• What happens: In peripheral tissues (like the liver and kidneys), T4 is converted to the more active T3 by the enzyme________.
• Why: T3 is the main active form of thyroid hormone, regulating metabolism at the cellular level.
iodothyronine deiodinase
Uptake of iodide
– inorganic iodide transported to follicular cell by_____ located in the basolateral membrane
NIS
Sodium Iodide Symporters
■ NIS activity modulated by_____
■ uptake also influenced by serum iodide levels
TSH - Thyroid Stimulating Hormone
- Oxidation of iodide
– Iodide then diffuses across the apical surface of the follicular cell to the lumen of the follicle via…
pendrin cotransporter
- Oxidation of iodide
– In the apical membrane, iodide is
oxidized to active iodine (I2) by_____
This reaction is catalyzed by the heme-containing enzyme_____
hydrogen peroxidase
thyroid peroxidase (TPO)
Iodide -> iodine
Hydrogen peroxidase
- Iodination of tyrosyl residues on Tg
– iodine is immediately incorporated into the tyrosine residues of the
large glycoprotein_______
molecules
thyroglobulin
- Coupling
– Iodinated thyroglobulin in the colloid of the follicle undergoes coupling of iodinated tyrosine molecules
– The reaction is catalyzed by_____
TPO
The Products of Coupling
■ The coupling of two tyrosine residues each iodinated at two positions (diiodotyrosine, T2 or DIT) produces___________
■ The combination of DIT with monoiodotyrosine (MIT or T1) produces
__________
tetra-iodothyronine or thyroxine (T4)
triiodothyronine (T3)
Storage and Release of T4 and T3
■ Thyroid hormones are stored in this state and released when the thyroglobulin molecule is taken up by a process of _______into the follicular cells via_____
pinocytosis (cell drinking)
megalin
Storage and Release of T4 and T3
■ This process is stimulated
by____ and the thyroglobulin
ends up in lysosomes
TSH
- Proteolysis of Tg
– Within______, hydrolysis of the
thyroglobulin molecules occurs and
results in the release of T4 and T3
– Both T4 and T3 are released into
circulation
lysosomes
- Deiodination of______ within the thyroid gland and reuse of liberated iodide
iodotyrosines
- Deiodination of T4 to T3
– about half of all T4 is monodeiodinated in the_____position to form T3
– 40% is deiodinated at the inner ring,
forming___
5’
rT3 - Reverse triiodothyronine
