The Thyroid gland Flashcards
anatomy of the thyroid
- Lies against and around front larynx and trachea
- Below thyroid cartilage (Adams apple)
- 2 lobes joined by an isthmus
- now tie shaped
- Isthmus extends from 2nd and 3rd rings of the trachea
parathryoid and thyroids are
distinct glands
embryological development of the thyroid gland
- First endocrine gland to develop
- At 3-4 weeks gestation it appears as an epithelial proliferation in floor of pharynx at base of the tongue and takes several weeks to migrate to final position
- First descends as diverticulum through thyroglossal duct and migrates downwards passing in front of hyoid bone
- During migration remains connected to tongue by thyroglossal duct which subsequently degenerates
- Detaches thyroid then continues to its final position over following 2 weeks
follicular cells are arrnaged in spheres called
thyroid follicles
thyroid follicles are filled with
colloid- a deposit of thyroglobulin
(extracellular even though inside follicle)
The thyroid and parathyroid are distinct glands
parafollicular cells (distinct from parathryoid principle cells) produce
calcitonin
thryoid hormone synthesis
- T3- triiodothyronine
- T4- tetraiodothyronine (thyroxine)
- 2 tyrosine linked together with iodine at three or four position on the aromatic ring
- Thyroglobulin acts as a scaffold on which thyroid hormones are formed
Thyroglobulin acts as
a scaffold on which thyroid hormones are formed
- occuring on tyrosine residues
dietary idoine is …… before absorption in the small intestine
reduced
Thyroid hormone and precursors are the only molecules in the body that contain
iodine
thyroid gland contains ……% of iodine in the body
90-95%
how is iodide taken up by thyroid epithelial cells
Iodide (I-), is taken up from blood by thyroid epithelial cells, which have a sodium iodide symporter (iodine trap)
sources of iodine
iodized salt, dairy products, grains and meat
thyroid peroxidase
Membrane bound enzyme that regulates 3 separate reactions involving iodine
name the three reactiosn thyroid peroxidase regulates
- Oxidation of iodide to iodine (requires H2O2)
- Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
- Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein
thyroid hormone synthesis
- Important amino acids imported from the plasma into the cell for thyroglobulin synthesis
- Thyroglobulin exported into colloid
- Iodide (recued dietary iodine) then imported into the cell from the plasma via the iodide trap (sodium iodide symporter)
- Iodide oxidised to iodine by thyroid peroxidase
- Iodination of thyroxine residues by thyroid peroxidase
- Coupling of tyrosine residues together by thyroid peroxidase
- Colloid taken into cell via pinocytosis to form a vesicle full of thyroglobulin
- Lysosome fuses with vesicle
- These enzymes degrades the thyroglobulin protein releasing thyroid hormone
- Deiodinase enzyme will recycle any unused iodine
T3/T4 ratio
90% of thyroid hormone secreted is T4- Most T4 is converted to T3 outside thyroid
difference between T3 and T4
T3 is ……more biologically active than T4
4x
where is most T4 converted to T3
in the liver and kidney
80% of ciruclating Thyroid hormone is
T3 derived from T4
how are T3 and T4 transported in the blood
thyroxine-binding globulin
regulation of thryoid hormone
negative feedback
- when thyroid hormoen is high it provides negartive feedback to both the AP (stop producing TSH) and the hypothalamus (stop producing TRH)
Thyroid hormone effect virtually every cell in the body and have 2 interconnected responses
- Effects on cellular differentiation and development
- Effect on metabolic pathways
Tyroid stimulating hormones (TSH)
- Glycoprotein hormone composed of 2 non-covalently bound subunits (alpha and beta)
- (Alpha subunit also present in FSH and LH)
- B subunit provides unique biological activity
effects of TSH
Triggers the release of thyroid hormone
TSH stimulates which axtions
Iodide uptake
Iodide oxidation
Thyroglobulin synthesis
Thyroglobulin iodination
Colloid pinocytosis into cell
Proteolysis of thyroglobulin
Cell metabolism and growth
general action of thryoid hromone
1) Increase in basal metabolic rate and heat production
2) Stimulation of metabolic pathways
3) Sympathomimetic effects
1) Increase in basal metabolic rate and heat production
- In most tissues (except the brain, spleen and testis), thyroid hormones stimulate the metabolic rate by:
- Increasing the number and size of mitochondria
- Stimulating the synthesis of enzymes in the respiratory chain
2) Stimulation of metabolic pathways
Catabolic pathways are generally stimulated more than anabolic
- Lipid metabolism: stimulates lipolysis and B-oxidation of fatty acids
- Carbohydrate metabolism: stimulates insulin-dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis
3) Sympathomimetic effects
Increases target cell response to catecholamines by increasing receptor number on target cells
Sympathomimetic effects on the cardiovascular system
- increases hearts responsiveness to catecholamines
- Increased CO (increase heart rate and force of contraction)
- Peripheral vasodilation to carry extra heat to body surface (B2 adrenergic receptors)
Sympathomimetic effects on the nervous system
Essential for both development and adult function
Increased myelination of nerves and development of neurones
thyroid hormone receptor is a
nuclear receptor
function of thyroid hormone receptor
- Modulate gene expression
- Thyroid hormone receptors bind DNA in the absence of hormone leading to transcriptional repression
- Hormone binding associated with a conformational change in the receptor that causes it to function as a transcriptional activator
normal plasma level of thyroid hromone
goitre
- Enlargement of the thyroid gland
- Develops when the thyroid gland is overstimulated
- May accompany either hypo- or hyperthyroidism (but not necessarily present in either)
signs of hypothyroidism
Low T3
Low T4
Elevated TSH
hypothyroidism in infants
cretinism
- dward stature
- mental deficiency
- poor bone dev
- slow pulse
- muscle weakness
- GI disturbance
hypothyroidism in adults
myxedema
- thick puffy skin
- muscl weakness
- slow speech
- mental deterioration
- intolerance to cold
autoimmune hypothyroidism
hashimotos
hyperthryoidism signs
- high T3
- high T4
- low TSH
autoimmune hyperthyroidism
graves disease
method used to scan the thyroid
thyroid scintigraphy
thyroid scintigraphy uses
Technetium-99m (99mTc) used for isotope scanning of the thyroid with a gamma camera.
biological halflife of Technetium-99m
1 day (radiation xpsoure low)
