THYROID GLAND

Question 1

Location and morphology of thyroid gland

Answer 1

The thyroid gland is a butterfly-shaped endocrine organ located in the lower neck, in front of the trachea, just below the larynx.

Question 2

Function of the thyroid gland

Answer 2

It plays a crucial role in regulating metabolism through the secretion of thyroid hormones (T3 and T4) and calcitonin.

Question 3

• Ring-shaped structure

Answer 3

Follicle / Follicular cells

Question 4

• Basic Unit: Spherical structures filled with colloid (a gel-like substance).

• Lined by follicular cells (simple cuboidal or columnar epithelium).

Answer 4

Thyroid Follicles

Question 5

Thyroid Follicles FUNCTION

Answer 5

Synthesize and secrete thyroid hormones (T3 and T4).

Colloid contains thyroglobulin, a precursor to these hormones.

Question 6

• Colloid (aka.____ or _____)
- hormones
- thyroglobulin
- glycoproteins

Answer 6

Cavity or Lumen

Question 7

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).

Answer 7

thyroglobulin

pink

Question 8

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).

Answer 8

thyroglobulin

pink

Question 9

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.

Answer 9

follicular

colloid; T3 and T4.

Iodine

Question 10

Colloid (aka. Cavity)

Synthesizes (3)

Answer 10

• hormones
• thyroglobulin
• glycoproteins

Question 11

Follicle
• cavity surrounded by simple cuboidal epithelium called…

Answer 11

Follicular Cells

Question 12

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

Answer 12

Bases

Apices

Question 13

Parafollicular Cells
• aka

Answer 13

C cells

Question 14

Parafollicular cells produces what?

Answer 14

Calcitonin

Question 15

Role of Calcitonin

• Target:
• Purpose:

Answer 15

Bones, kidneys, and intestines.

To lower blood calcium levels when they are too high.

Question 16

Roles of calcitonin

• : Prevents osteoclasts (bone cells that break down bone tissue) from releasing calcium into the bloodstream.

• : Increases calcium excretion in the kidneys.

Answer 16

Inhibits Bone Resorption

Promotes Calcium Excretion

Question 17

Thyroid Hormone
Synthesis
•_____
- major component of thyroid hormone

Answer 17

iodine

Question 18

Sources of Iodine

Answer 18

• dietary intake
• deiodination of organic iodine-containing moieties within the gland

Question 19

glycoprotein synthesized by the rough endoplasmic reticulum in the basal and perinuclear regions of the follicular cell

Answer 19

TGB or Tg

Question 20

• Thyroglobulin (TGB or Tg)
- modified by the______, packaged into_____

• vesicles undergo_____, releasing Tg into the lumen

Answer 20

Golgi complex

vesicles

exocytosis

Question 21

Thyroid Hormone Synthesis

1-7

Answer 21

1. Uptake of lodide
2. Oxidation of lodide
3. lodination of the tyrosyl residues on Tg
4. Coupling
5. Proteolysis of Tg
6. Deionidation of iodotyrosines
7. Deionidation of T, to T3

Question 22

1. 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.

Answer 22

follicular cells

sodium-iodide symporter (NIS)

Question 23

2. 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.

Answer 23

thyroid peroxidase (TPO)

follicular lumen (the colloid).

Question 24

3. 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.

Answer 24

monoiodotyrosine (MIT) and diiodotyrosine (DIT),

Question 25

4. 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.

Answer 25

• T3 (triiodothyronine): MIT + DIT • T4 (thyroxine): DIT + DIT

Question 26

5. 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.

Answer 26

Lysosomal enzymes

Question 27

6. 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

Answer 27

iodotyrosine deiodinase

Question 28

7. 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.

Answer 28

iodothyronine deiodinase

Question 29

Uptake of iodide – inorganic iodide transported to follicular cell by_____ located in the basolateral membrane

Answer 29

NIS Sodium Iodide Symporters

Question 30

■ NIS activity modulated by_____ ■ uptake also influenced by serum iodide levels

Answer 30

TSH - Thyroid Stimulating Hormone

Question 31

2. Oxidation of iodide – Iodide then diffuses across the apical surface of the follicular cell to the lumen of the follicle via…

Answer 31

pendrin cotransporter

Question 32

2. Oxidation of iodide – In the apical membrane, iodide is oxidized to active iodine (I2) by_____ This reaction is catalyzed by the heme-containing enzyme_____

Answer 32

hydrogen peroxidase thyroid peroxidase (TPO)

Question 33

Iodide -> iodine

Answer 33

Hydrogen peroxidase

Question 34

3. Iodination of tyrosyl residues on Tg – iodine is immediately incorporated into the tyrosine residues of the large glycoprotein_______ molecules

Answer 34

thyroglobulin

Question 35

4. Coupling – Iodinated thyroglobulin in the colloid of the follicle undergoes coupling of iodinated tyrosine molecules – The reaction is catalyzed by_____

Answer 35

TPO

Question 36

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 __________

Answer 36

tetra-iodothyronine or thyroxine (T4) triiodothyronine (T3)

Question 37

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_____

Answer 37

pinocytosis (cell drinking) megalin

Question 38

Storage and Release of T4 and T3 ■ This process is stimulated by____ and the thyroglobulin ends up in lysosomes

Answer 38

TSH

Question 39

5. 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

Answer 39

lysosomes

Question 40

6. Deiodination of______ within the thyroid gland and reuse of liberated iodide

Answer 40

iodotyrosines

Question 41

7. 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___

Answer 41

5’ rT3 - Reverse triiodothyronine