Thyroid Flashcards
gross anatomical structure of thyroid gland
anterior to the trachea and slightly inferior to the larynx
the thyroid gland is composed of two lateral lobes and a central isthmus
the isthmus lies anterior to the 2nd and 4th tracheal rings
enclosed in pre-tracheal fascia which anchors thyroid to the trachea and it therefore moves on swallowing
development of thyroid gland
- A thyroid diverticulum forms in the midline of the floor of the mouth between the 1st and 3rd brachial arch components of the developing tongue
- it grows caudally over the developing larynx to the anterior aspect of the trachea
- as it descends it associates with the superior/inferior parathyroids which develop from the 4th/3rd pharyngeal pushes and with neural crest cells which will form the parafollicular C calcitonin cells
- two lateral loves and a central isthmus forms. Tissue along the line of descent usually disappears. However, ectopic thyroid tissue can be found in the tongue, along the line of descent, and in the thorax
thyroid gland histological structure
epithelial cells of the thyroid, follicular cells, are arranged in follicles around a lumen filled with colloid
surrounded by parafollicular cells- form calcitonin
Control of thyroid hormone secretion explained
The hypothalamus releases thyrotrophin releasing hormone which controls thyroid stimulating hormone release from the anterior pituitary.
TRH is released in response to changes in plasma glucose and core temperature.
Hyperglycaemia stimulates, hypoglycaemia inhibits
cold stimulates, warmth inhibits
The production of TSH in thyrotroph cells controls T3 (triiodothyronine) and T4 (thyroxine) secretion by thyroid follicle cells
negative feedback on T3 and T4 on TSH production
What does the production of T3 and T4 require?
iodide
What makes the thyroid differ from other endocrine glands?
Store large amounts of hormone precursor extracellularly
What synthesises the thyroglobulin and where is it released?
Cuboidal follicular cells, and then release into colloid
How do the active and inactive follicles differ?
active epithelial cells are longer, more columnar and the colloid is reduced in size
inactive glands have low cuboidal cells and follicles are filled with colloid
What is thyroglobin + function?
a glycoprotein, that forms 50% of the mass of protein within the thyroid
iodide molecules bind to tyrosine residues present and are subsequently cleaved, forming thyroid hormones
Where are parafollicular C cells located? + function
located in the base of the follicle epithelium
secrete peptide hormone calcitonin which lowers raised plasma calcium
How are T4 and T3 produced?
- sodium/iodide symporter is present on the basal membrane of follicular cells. It traps and pumps iodide from plasma
- thyroperoxidase enzyme on the apical plasmalemma oxidises the iodide to iodine, iodinates tyrosyl resides in the thyroglobin and couples tyrosyl residues to produce hormone T3 and T4, still bound to the thyroglobin, hence inactive
- TSH stimulates endocytosis of colloid and its digestion by lysosomes, freeing T4 and T3
- TSH acts via cAMP
What is the source of iodide? + importance
seaweed, dairy, fish
iodine deficiency can prevent formation of T4 and T3
Plasma transport explained
T3 and T4 circulate bound to plasma proteins and thus have a longer half life
binding proteins include: thyronine-binding globulin which is a glycoprotein with a higher affinity for T4 than T3, produced by the liver
transthyretin- lower affinity for T4 than T3
albumin- very low affinity but high capacity
How is the active form of iodothyronine formed?
peripheral metabolism produces active T3 from circulating T4
main thyroid product T4 is not metabolically active, T3 is the active form.
Metabolism of T4 produces metabolically active T3
- T4 converted into T3 by Type I-deiodinase, providing T3 to plasma
- T4 is also converted into inactive rT3 by type II deiodinase, important for negative feedback of T4, providing intracellular T3 in the brain, pituitary and adipose
- deionisation is controlled by the need for metabolism. it is reduced in starvation due to raised cortisol, in severe illness and by propranolol
What happens upon breakdown?
iodothyronines are deiodinated to thyronine
iodide is salvaged by the kidney and reused
