14.1 - Thyroid Gland Flashcards
Structure and placement of the thyroid gland
- Lies against and around front of larynx + trachea
- Thyroid glands situated just above cricoid cartilage and below the thyroid cartilage
- There is a right and a left lobe of the thyroid, and joined by a structure called isthmus
- Isthmus extends from 2nd to 3rd rings of trachea
- Found near C7 of the spine
- ‘bow tie’ shape and location
Embryological development of thyroid gland
- First endocrine gland to develop
- 3-4 weeks gestation it appears as an epithelial proliferation at base of tongue (floor of pharynx)
- Takes several weeks to migrate into final position
- Descends through thyroglossal duct and migrates downwards in front of hyoid bone
- During migration remains connected to tongue by thyroglossal duct which then degenerates
- Detached thyroid then continues to final position
- If migration doesn’t occur as normal, thyroid will be in different place
Histology points of thyroid tissue
- Follicular cells arranged in spheres called thyroid follicles
- Follicles filled with colloid
- Colloid is a deposit of thyroglobulin
- Thyroglobulin is where thyroid hormone is sythesised
- Colloid is extracellular (even though it is inside follicle)
- Thyroid gland also contains larger parafollicular cells. These don’t form spheres of colloid. These produce calcitonin
How are thyroid hormones produced
- Produced on protein called thyroglobulin
- Thyroid hormones derived from tyrosine residues that are iodinated (ie have iodines added)
- Tyrosine residues within thyroglobulin undergo iodination (either one or two iodines are added)…
☞ 1 iodine added = monoiodotyrosine (MIT)
☞ 2 iodines added = diiodotyrosine (DIT) - Coupling of MIT + DIT = T3 or DIT + DIT = T4
- Thyroid peroxidase is vital for this process (different card)
What do T3 + T4 consist of (how they are produced on different card)
T3 aka triiodothyronine is made of one MIT and one DIT coupled together
T4 aka tetraiodothyroinine aka thyroxine is made of two DIT coupled together
T3 + T4 are thyroid hormones
Thyroglobulin
- Produced by parafollicular cells
- Acts as a scaffold on which thyroid hormones are formed
- Contains many tyrosine molecules (which are needed for thyroid hormone synthesis)
- Only a handful of these are used to synthesise T3 + T4
What does thyroid peroxidase do
- Multipurpose enzyme involved in thyroid hormone synthesis
- oxidation of iodide (present in cells) → iodine, requiring the presence of H2O2
- iodination where iodine is added to a tyrosine residue on thyroglobulin protein
- coupling of MIT or DIT to generate thyroid hormones within thyroglobulin
Dietary iodine
- Dietary iodine reduced → iodide before absorbtion, so that it can be stored in cells. This reduction occurs in small intestine
- Iodide is oxidised back to iodine when it needs to be used by thyroglobulin + thyroid peroxidase
- Thyroid hormones and precursors are the only molecules in human body that contain iodine
- Iodide is taken up from blood by thyroid epithelial cells that have a sodium-iodide symporter aka iodine trap
- Dietary sources of iodine are mainly near sea (eg crops grown near sea) and seaweed
- Also found in dairy, grains, meat and some in veg + egg. Iodized salt is best source
Why is most T4 is converted to T3 outside thyroid
- The main product from the thyroid gland is T4 but T3 has much more biological activity
- Most T4 → T3 in the liver and kidneys
How are T3 + T4 transported around the body
- These hormones are lipid soluble
- Therefore cannot be easily transported around in aqueous environment
- Transported in blood bound to the protein thyroxine-binding globulin
Regulation of thyroid hormone
hypothalamus releases TRH → causes anterior pituitary to release TSH → TSH stimulates thyroid gland to produce thyroid hormone (T3 + T4) → thyroid hormone has affects on target tissues
Regulated by negative feedback
☞ thyroid hormone has negative (inhibiting) effect on anterior pituitary + hypothalamus via long loop
☞ TSH has negative effect on hypothalamus via short loop feedback
What does thyroid hormone have an effect on
- Thyroid hormone = T3 + T4
- Effect virtually every cell in the body
- More general effects rather than specific
- Two main interconnected responses:
☞ metabolism
☞ growth and development
Thyroid stimulating hormone (TSH) + what does it stimulate
- Glycoprotein hormone composed of 2 non-covalently bound subunits
- Consists of an α and a β subunit
- Triggers intracellular signalling pathways → **thyroid gland stimulated to produce thyroid hormones… therefore
☞ iodide uptake
☞ iodide oxidation
☞ thyroglobin synthesis
☞ thyroglobulin iodination
☞ colloid pinocytosis into cell
☞ proteolysis of thyroglobulin
☞ cell metabolism and growth
What differentiates LH, FSH and TSH from each other
They all have two subunits:
- α subunit all the same present in all three
- β subunit differs between them and provides unique biological activity
What is another name for T4 hormone
thyroxine
