1/3 Thyroid Gland - Thyroid Hormone Regulation COMPLETE*** Flashcards
Thyroid Hormone: Embryologically, where does the thyroid originate from?
it is down growth from the pharyngeal ectoderm of the developing tongue
Thyroid Hormone: Describe the structure of the thyroid gland 5
it consists of thyroid follicles
the follicles vary in size
the lumen is filled with colloid
a single layer of follicular cells lines the follicles
microvilli are present on the follicular-colloid interface
Thyroid Hormone: What other cell type is present in the connective tissue near follicles?
- What do these cells do?
parafollicular cells
they secrete calcitonin
Thyroid Hormone: What is the function of the Microvilli?
- What is the function of the Dense capillary network?
to increase the surface area
to deliver nutrients to follicular cells and transport hormones
Thyroid Hormone: Describe the process by which Thyroid hormones are synthesised
iodine is circulating in the blood
iodine uptake is stimulated by thyroid stimulating hormone
it is transported across the follicle cell basal membrane with sodium
this is against its concentration gradient by secondary active transport
follicular cells produce lots of the protein thyroid globulin
this protein is rich in tyrosine and amino acids
thyroid globulin is transported into the colloid by exocytosis
iodide is oxidised to iodine and hydrogen peroxide is reduced
this is catalysed by TPO in the apical membrane
iodine is added to tyrosine within thyroglobulin as they are secreted into the colloid
this is catalysed by iodinase
Thyroid Hormone: Describe how T3 and T4 in the colloid are secreted into circulation
T3 and T4 are bound to thyroglobulin
they are packaged into vesicles
the vesicles are transported into follicular cells by endocytosis
lysosomes fuse with the vesicle
enzymes released separate T3 and T4 from thyroglobulin
free T3 and T4 are lipid soluble so diffuse across the basal membrane into the interstitium
they diffuse form the interstitium into the blood
most bind to proteins for transport in the blood
Thyroid Hormone: What is the function of follicular cells?
- What is the actual name for T3?
- What about T4?
- How much of each are produced?
to produce thyroid hormones T3 and T4
tri-iodothyronine
thyroxine
90% is thyroxine
Thyroid Hormone: Describe the composition of Thyroid hormones 2
they are derived from tyrosine amino acids
they are formed by the linkage of iodinated tyrosine molecules
Thyroid Hormone: When in the blood, why are most thyroid hormones bound to proteins?
- Which proteins are they bound to? 2
- Which hormone is biologically active?
because they are lipid soluble
albumin and thyroid binding globulin
only the free hormone
Thyroid Hormone: Why is a greater proportion of T3 free in the blood compared to T4?
because T3 has a lesser affinity for proteins
Thyroid Hormone: Describe what happens to T4 when it is a free hormone 3
it enters cells
it is deiodinated to T3
any T4 that isn’t deiodinated to T3 will be deiodinated to reverse T3
Thyroid Hormone: Explain how the process of Deiodination is regulatory 2
T3 is biologically active but rT3 is inactive
so conversion of T4 to rT3 slows metabolism
Thyroid Hormone: What is the general action of Thyroid hormones?
- What do they do during illness or starvation and why? 4
they increase basal metabolic rate and maintain body temperature
less TSH is released from the pituitary gland
so less thyroid hormone is produced
deiodination is altered such that more rT3 is formed from T4
this lowers basal metabolic rate to conserve energy
Thyroid Hormone: Once inside a cell, how do thyroid hormones exert their action? 2
it binds to DNA
it increases or decreases transcription
Thyroid Hormone: How do you get rid of thyroid hormone once used? 2
- Where does this take place?
it is deiodinated to di-iodotyrosine and mono-iodotryrosine
iodine is recycled or excreted in urine
in the liver and kidneys
