Thyroid gland Flashcards
Where is the thyroid gland?
just below the cricoid cartilage
What structure joins the two lobes of the thyroid gland?
the isthmus
Describe the embryological development of the thyroid gland
- first gland to develop
- starts as epethilial proliferation in floor of pharynx at the base of the tongue at 3-4 weeks
- then descends through the thyroglossal duct and passes in front of the hyoid bone
- It remains connected to the thyroglossal duct during its migration but this then degenerates
Describe the histology of the thryroid gland
- follicular cells arranged in spheres called thyroid follicles
- the follicular cells secrete colloid (a deposit of thyroglobulin) into the follicles for storage
What is T3 and T4 made of?
T3- monoiodotyrosine (MIT) + diodotyrosine (DIT)
T4- DIT +DIT
MIT and DIT is tyrosine residues with thryoglobulin and iodine added, MIT has 1 iodine, DIT has 2, T3 has 3 and T4 has 4
Which thryoid hormone is secreted more from the thryroid gland and which is most active?
T4 is secreted more but T3 is more active
Where can T4 be converted to T3?
liver and kidneys
how does T3 and 4 travel in the blood?
bound to thyroxine binding globulin
Are thyroid hormones the only substances in the body to use iodine?
yeh
How is iodide taken up from blood into thyroid epithelium?
a sodium- iodide symporter
Describe synthesis, storage and release of thyroid hormone
- iodide (I-) taken up by iondine- sodium symporter
- tyrosine taken up from blood also and made into thyroglobulin
- iondide and thryoglobulin excreted into follicles
- iodide converted to iodine (I) by oxidation
- iodine and thryoglobulin bound by iodination to make MIT/DIT which are coupled to make T3/4
- pinocytosis of T3/T4
- lysosome fuses with vesicle with T3/4 in
- some T3/4 released, some degraded to release I- for more synthesis
Describe endocrine control of thyroid hormone release
- hypothalamus secretes TRH
- this travels by portal system to anterior pituitary where it stimulates release of TSH
- TSH travels in blood to thyroid gland where in stimulates synthesis and release of thyroid hormone
- t3/4 inhibts release of TSH and TRH when levels too high
- TSH inhibits release of TRH when its levels are too high
What is the difference in structure of TSH, FSH and LH?
TSH, FSH and LH all have an alpha unit which is the same but the beta unit differs and provides specificity
What type of receptor does TSH bind to?
an s or q GPCR- both stimulate release of T3/T4
Describe the intra cellular process that occurs when TSH binds to a GPCR
if S type: - adenylyl cyclase activated - cAMP produced - PKA activated if Q type: - phosphlipase C activated - PIP2--> DAG and IP3 - causes Ca2+ release via Ip3 receptor - and PKC activation
both routes result in thyroid hormone synthesis and release
What general effects do thyroid hormones have?
- increase basal metabolic rate (except in brain, spleen and testis) by stimulating more and larger mitochondria and synthesis of enzymes in the respiratory chain
- stimulates catabolic pathways- lipid metabolism and more insulin intake meaning more gluconeogenesis and glycogenolysis
- increase catecholamine receptor synthesis so cells increase sympathetic output
What effects do thyroid hormones have on CVS?
- increase sensitivity to sympathetic stimulation
- increase peripheral vasodilation to reduce temperature
What effects does thyroid hormone have on the CNS?
- essential for development and adult function
- increases myelination of nerves for development of neurones
Where do thyroid hormones (amino acid derivatives) act?
in the nucleus - bind to hormone response elements to increase regulation of certain genes
What is the role of thyroid peroxidase?
- oxidses iodide to iodine
- adds iodine to tyrosine
- couples MIT and DIT
What are the symptoms of hypothyroidism?
- excessive tiredness, feeling down
- memory problems, depression, phycosis
- weight gain
- cold intolerance
- gruff, croaky voice
- puffy eyes, hands and feet
- dry, flaky skin
- hair loss (outer third of eye brows)
- carpal tunnel symptoms
- mennorrhagia
- muscle weakness and cramps
What are the signs of hypothyroidism?
- may be none
- pallor (peaches and cream face)
- coarse facial featurea and perioorbital puffiness
- bradychardia
- non pitting odema- myxoedema
- ascities or pericardial effusion (very rare)
What are the common causes of pitting vs non pitting oedema?
pitting: water retention: heart failure, varicose veins, pregnancy ect
non pitting: lymph node dysfunction, myxodema in hypothyroidism
What is the most common cause of hypothyroidism in the UK?
- hashimotos disease
- auto immune destruction of thryoglobulin and thyroid perioxidase in blood. In early stages there may be a small diffuse goitre but this may not happen
- more common in women
How is hypothyroidsm treated?
oral T4 (not broken down in intestine)
What is the most common cause of hypothyroidism globally? What other effects come with this cause?
severe iodine deficiency
- leads to a usually nodular goitre
- if in children leads to cretinism as well as hypothyroidism: mental retardation, deaf, mute, short stature due to developmental impairment
What is the difference between thyrotoxicosis and hyperthyroidsm?
Hyperthyroidism (over active thyroid gland) always leads to thyrotoxicosis (high thyroid hormone). But there are other causes of thyrotoxicosis.
What are the symptoms of hyperthyroidism?
- overactivity, tiredness
- nervouseness, anxiety, insomnia
- shaking, trembelling
- heart intolerance
- increased sweating- warm sweaty hands
- palpitations, rarely angina
- weight loss inspite of increased appetite
- diarrhoea
- amenorrhea
- proximal muscle weakness
What are the signs of hyperthyroidism?
- weight loss
- warm sweaty hands
- fine hand tremour
- tachycardia
- AF - irregular pulse
- bounding pulse
- proximal myopathy
- lid lag (delay in eyelid movement down when eye moves down due to more sympathetic stimulation)
- staring eyes
How are the signs of anxiety different to hyperthyroidism?
- cold and sweaty hands
- weight loss is due to eating less
- history
- otherwise symptoms similar
Why is it rare for a pituitary adenoma (affecting TSH production) to present with hypo or hyperthyroidism?
- not hyperthryoidism because its very rare for it to secrete more TSH
- not hypothyroidism because if adenoma is affecting hormone release then other stuff is likely to go more wrong more quickly
What is measured in blood to diagnose hyper or hypothyroidism?
TSH- if high then hypothroidism as the gland is underfunction so less T3/4 so more TSH release
What are the 3 key causes of hyperthyroidism?
- graves disease
- toxic multi nodular goitre
- toxic adenoma of thyroid gland
What is graves disease and what are key differentiating signs?
- auto immune condition producing thyroid stimulating immmunoglobulin so more T3/4 released
- exopthalmos (bulding eyes) and pretibial myxoedema (not in every case)
- sometimes a diffuse goitre
Why is myxodema found in hypothyroidism?
deposition of mucopolysaccharides
when and how does a toxic multinodular goitre develop?
- low T4 from mild iodine deficiency
- thryoid cell hyperplasia to adjust
- increased replication predisposes TSH receptor mutation
- TSH receptor mutation makes it consitiutively active
What will a toxic adenoma of the thyroid gland look like on a radioactive iodine scan?
- looks like one lobe/ area much larger than the other
- as the adenoma takes up most of the iodine and surpresses most of the gland
How is hyperthyroidism treated?
- carbimazol
- prevents thyroid peroxidase coupling iondinated tyrosines
- so less T4 production
What is the prognosis of adenoma of thyroid gland?
97% cure rate
1% malignancy rate
should the thyroid gland be able to be felt or seen?
no
Why does the thyroid gland normally move up and down on swallowing?
- it is enclosed in the pre-tracheal fascia which also attaches to the trachea and larynx (and thryroid cartilage)
- this moves it moves up on swallowing with the thyroid cartilage
How can the thyroid gland be observed?
- ultrasound scan
- technetium scinitgraphy: technetium is similar to iodine but radioactive so can be detected, this can give impression of size and activity of thyroid
What is a thyroglossal duct cyst?
- when the thyroglossal duct persists after birth and fills with fluid (creates cyst)
- creates bulge on/ near midline near/ under hyoid bone
- will move up when you stick your tongue out due to where it attaches to mouth floor
WHat are the three types of goitre?
- diffuse (both nodes enlarge equally)
- multinodular (many nodules to goitre)
- single nodular
When are small goitres normal?
- pregnancy
- menopause
- after/ during a girls first period
What is the most common cause of goitre globally and in UK?
- globally: iodine deficiency
- Uk: multinodular goitre of unknown cause: normal thryoid function but after many years it may lead to toxic multinodular goitre and so cause hypertyroidism
What is a major complication of goitres?
- they enlarge backwards
- cause retrosternal goitre
- cause tracheal compression
