thryoid gland

where located?

basic structure?

embryological origin?

below larynx

has two lobes & an isthmus in the middle.

comes from endoderm fr floor of pharynx at the foramen cecum

it descend infront of the gut as a bilobed structure temporarily attached to thyroglossal duct

moves into place @ week 7

circular structures in thyroid?

contain what?

surrounded by what?

circular thyroid follicles

contain colloid

lined by simple epithelium (from squamous to cuboidal to columnar)

parafolicular cells (C-cells)

derived from?

where located?

derived fr neural crest cells that migrate to Ultimobranchial body of 5th pharyngeal pouch then C-cells migrate to thyroid gland

these cells don't touch the thyroid follicles

thyroid follicular cells

secrete what? 

where are these stored?

secrete T4 & T3

stored in colloid

parafollicular cells

what secrete? what does it act on?

3 effects of this

high Ca in blood incr what? decr what?

secrete calcitonin which act on osteoclasts

1. inhib bone resorption (cutting up bone)

2. raise Ca absorption by bone

3. lower Ca & PO4 in blood

high Ca therefore incr calcitonin release & decr PTH release

how thyroid makes T4 & T3

5 steps

1. TSH fr pituitary bind @ basal surface of follicular cells.

2. TGB synth via RER & Golgi & excreted to lumen of follicle

3. Iodine is actively t/p via I/Na pump into basal surface of cell.

4. Thyroid Peroxidase convert I to I² 

5. I² bind to tyrosine in TGB to make T3 & T4

diet low in iondine

thyroid can't make hormones -> hypothyroidism

TSH not inhibited by neg feedback thus still released -> cause hyperthyroidism

hyposecretion of thyroid hormones during fetal life. 

result in dwarfism & mental retardation

Thyroid hyperfunction due to immunological dysfunction.

Antibodies bind on TSH receptors & mimic TSH

thus thyroid is constantly stimulated

-> elevated thyroxine levels, bulging eyes, enlarged thyroid