Histology - Thyroid Flashcards Preview

BMP - Not SGU > Histology - Thyroid > Flashcards

Flashcards in Histology - Thyroid Deck (16):

Thyroid follicular epithelial cells are

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


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 


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)


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

A image thumb

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


Steps in Synthesis of Thyroid Hormones

2. Iodide trapping:

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


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


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]


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


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


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


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


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


Undiagnosed / untreated congenital hypothyroidism results in

irreversible failure of CNS maturation (cretinism).


Grave’s Disease


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).

A image thumb


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.


Parafollicular or C-cells

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