Histology - Thyroid Flashcards Preview

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Flashcards in Histology - Thyroid Deck (16):
1

Thyroid follicular epithelial cells are

A simple cuboidal epithelium arranged in spherical follicles 200-300 mm in diameter.

2

Synthesis of T3 and T4 occurs in part intracellularly and in part extracellularly.

The hormones are stored extracellularly attached to ________ in _______ until the gland is stimulated to secrete. 

thyroglobulin in colloid 

3

Follicular cells are seen as a __________ epithelium surrounding the follicles.The height of the epithelium increases (______________) as follicles become more active and decreases as follicles are less active

simple cuboidal epithelium

(cuboid → columnar)

4

List the 8 steps in the synthes is of thyroid hormones.

  1. Thyroglobulin synthesis
  2. Iodide trapping
  3. Oxidation
  4. Organification
  5. Coupling
  6. Endocytosis
  7. Secretion
  8. Recycling

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5

Steps in Synthesis of Thyroid Hormones

1. Thyroglobulin synthesis:

Thyroglobulin (TG), a glycoprotein containing large quantities of tyrosine, is synthesized on the RER and Golgi apparatus of follicular cells → secreted across the apical membrane into the follicular lumen

6

Steps in Synthesis of Thyroid Hormones

2. Iodide trapping:

Iodide (I - ) is actively transported (sodium iodide symporter) from blood into follicular epithelial cells.

7

Steps in Synthesis of Thyroid Hormones

3. Oxidation:

After transport into the cell, iodide traverses the cell to the apical membrane → oxidized to a reactive species by thyroid peroxidase and transported into the follicle lumen

8

Steps in Synthesis of Thyroid Hormones

4. Organification:

Just inside the lumen, iodine combines with the tyrosine in TG (2nd reaction catalyzed by thyroid peroxidase) → monoiodotyrosine (MIT) and diiodotyrosine (DIT)

[MIT and DIT remain attached to TG]

9

Steps in Synthesis of Thyroid Hormones

5. Coupling:

Two possible coupling reactions occur between MIT and DIT

(3rd reaction catalyzed by thyroid peroxidase) 

 

1) 2 molecules of DIT combine to form T4

OR 

2) DIT combines with MIT to form T3. DIT-DIT is the favored “coupling” reaction, more T4 is produced than T3.  

TG now contains T4, T3, and uncoupled MIT and DIT

10

Steps in Synthesis of Thyroid Hormones

6. Endocytosis:

When the gland is stimulated (normally by pituitary TSH) → iodinated TG (T4, T3, MIT, and DIT) is brought into the follicular cells by endocytosis → transported in the direction of the basal membrane along microtubules

11

Steps in Synthesis of Thyroid Hormones

7. Secretion:

Endocytotic vesicles fuse with lysosomes → hydrolyze peptide bonds releasing T4, T3, MIT, and DIT →T4 and T3 are secreted across the basal membrane → enter capillaries → systemic circulation

12

Steps in Synthesis of Thyroid Hormones

 

8. Recycling:

MIT and DIT remain in the follicular cell → deiodinated inside the follicular cell by thyroid deiodinase → iodide is recycled into the intracellular pool → tyrosine molecules are recycled into synthesis of new TG → another cycle

13

Undiagnosed / untreated congenital hypothyroidism results in

irreversible failure of CNS maturation (cretinism).

14

Grave’s Disease

Hyperthyroidism

Thyroid- stimulating immunoglobulins (IgG) bind to TSH receptors and have the same effect on the TSH receptors as TSH itself (increased cAMP)

→ results in continuous stimulation of the thyroid gland, just as if TSH levels were continuously elevated

→ increased T3/T4 synthesis and secretion, hypertrophy/hyperplasia of the gland

→ hyperthyroidism and goiter (enlarged thyroid).

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15

Hypothyroidism

List four causes.

  1. Dietary iodine deficiency – cannot synthesize thyroid hormone
  2. Lack of negative feedback results in persistent elevation of TSH. Thyroid follicular cells respond by growing/dividing and storing thyroglobulin in follicles → diffuse enlargement of the thyroid gland or goiter
  3. Inflammation of the thyroid gland (thyroiditis) can also lead to hypothyroidism. Hashimoto’s Thyroiditis is a common form of thyroiditis.
  4. Thyroidectomy (obviously) causes hypothyroidism.

16

Parafollicular or C-cells

  • Larger, pale-staining cells found adjacent to / between follicles
  • Neural crest origin
  • Synthesize/secrete calcitonin