The Thyroid Gland Flashcards
The thyroid gland location
Within neck, surrounding trachea
How many lobes does the thyroid gland have?
2
What is the thymus made up of?
Multiple follicles made up of follicular sides round the outside and colloid in the centre
Parafollicular cells
Parathyroid glands are embedded where?
Which nerve runs close to the thyroid and what does it supply?
Within the thyroid
Left current laryngeal nerve runs close and supplies vocal cords
Where does the thyroid gland start and how does it develop embryologically?
Floor of pharynx (base of tongue) Develops into thyroglobulin duct Divides into 2 lobes Duct disappears leaving foramen caecum Final position by week 7 Thyroid gland then develops
What is colloid?
Sticky mucous in middle of follicles where the thyroid hormone is made
How is thyroid hormone made?
TSH released by anterior pituitary gland binds TSH receptor (thyroid stimulating hormone) adjacent to blood vessel.
Iodide ions are essential
Through sodium iodide transporters it is transported into follicular cell and then into colloid
TPO is also transported into follicular cell and into colloid
Iodisation makes iodine in the colloid
Thyroglobulin (prohormone) contains
Tyrosine residues which can be ionidated - stick iodine onto it, giving MIT and ….. which take part in a coupling reaction to produce hormones T3 and T4.
T3 and T4 are still bound to thyroglobulin, enter follicular cell and then blood circulation
What does MIT stand for?
What does DIT stand for?
What is produced if MIT and DIT are joined together?
What about if two DIT are joined together?
3-monoiodotyrosine
3,5-diiodotyrosine
3,5,3 triiodothyronine
3,5,3’,5’ tetraiodothyronine
Which is the main product of the thyroid gland?
In target tissues what happens?
T4
Deiodinated to form T3.
How is T4 (thyroxine) a prohormone converted into T3 (triiodothyronine)?
Deiodinase enzyme
Where is thyroglobulin made?
What is thyroid binding globulin?
Only in thyroid gland, it is a prohormone that forms T3 and T4
Plasma protein found in blood
What percentage of thyroid hormones are unbound?
What does this mean?
1%
Only 1% are active
Why do we need thyroid hormone?
Fetal growth and development
In particular that of the central nervous systemi
Increases basal metabolic rate
Protein, carbohydrates and fat metabolism
Potentiate actions of catecholamines (eg. Tachycardia,
Iipolysis)
Effects on GI, CNS, reproductive systems
What is untreated congenital hypothyroidism called?
Cretinism
What test is done to measure thyroid stimulating hormone?
Heel-prick test measures TSH in new borns
If it is high, the baby is not producing enough of its own thyroid hormone
Half life of T4?
Half life of T3?
7-9 days
2 days
How is thyroid hormone production controlled?
Thyrotropin releasing hormone is released by hypothalamus
Goes to anterior pituitary and stimulates thyrosome cells which release thyroid stimulating hormone
Thyroid releases T3 and T4
Creates a negative feedback loop - T3 and T4 negatively inhibit release of TRH and TSH
SS inhibits release of TSH
How much more common is thyroid disorders in women than men?
Why do we think this is?
4:1
Autoimmunity - antibodies made to various parts of your thyroid gland which destroy it, women are more at risk because of pregnancy and the way their immune systems have had to adapt to the foetus.
Is an overactive or underactive thyroid gland more common?
They’re the same
What can cause primary hypothyroidism
Autoimmune - destroys part of thyroid
Surgery - remove part of thyroid
if you take out/ destroy part of it, negative feedback loop changes, TSH goes up to produce more thyroxine and T4 and T3 but you can’t make the T4 so T4 goes down.
Always measure TSH and T4 - TSH goes up, T4 goes down
Describe some symptoms and signs of someone with hypothyroidism
Lethargic - feeling tired Depression Deepening voice Swelling around eyes Cold intolerance Weight gain Reduced appetite Constipation Bradycardia Weakness Dry skin Hypothermia Low libido Problems with periods Eventual myxoedema coma
What does levothyroxine do?
Produces T4
Even if the thyroid is taken out
All the target tissues contain deiodinase enzymes which convert T4 to T3
Why do we give levothyroxine to some people with hyperthyroidism
Block their thyroid hormone production and replace it with levothyroxine ie. only the minimum that they need
How do you adjust the dose of levothyroxine?
Adjusted according to TSH (aim to get into normal range)
Common does 100 micrograms
Given orally
Potential complications of levothyroxine?
Weight loss, headache
Rapid heart rate, heart attack
Combined thyroid hormone replacement
What can it cause?
Combination T4 and T3 - found on market without prescription
Toxicity
What happens during hyperthyroidism?
What are the symptoms
Thyroxine levels rise
TSH levels drop
Nervousness Irritability Insomnia Depression Anxiety Broken hair, hair loss Increased heart rate Weight loss despite increase in appetite Arrhythmia High blood pressure Weight loss Strong feeling of hunger Diarrhoea Muscle cramps Muscle weakness Warm, moist skin, increased body temperature Palpitations Nodules within thyroid gland Enlarged thyroid gland
Causes of hyperthyroidism?
Grave’s disease - whole gland smoothly enlarged and whole gland overactive
Toxic multinodular goitre
Solitary toxic nodule
What is Graves’ disease?
What happens?
What are the symptoms?
Autoimmune disease
Antibodies bind to and stimulate TSH receptor in the thyroid
Smooth goitre (enlargement of thyroid gland)
Other antibodies bind to muscles behind eye
This is exophthalmos - you can see white ring around iris and a bulging eyeball and the whites of the eye become red and angry looking
Other antibodies stimulate growth of soft tissue of shins
This is pretibial myxoedema - thickening and growth of shins
What connects the two lobes of the thyroid gland?
Isthmus
Where is the thyroid gland’s embryological origin?
Floor of pharynx
Which form of the thyroid hormone is active and which is inactive?
Which is the prohormone?
T4
T3
Thyroglobulin - produced by follicular cells combined with iodine from blood to create T3 and T4 in presence of thyroperoxidase
T4–> T3 in presence of deiodinase
Hyperthyroidism causes
Autoimmune (Grave’s disease) or nodule- related
What connects the two lobes of the thyroid gland?
Isthmus
Where is the thyroid gland’s embryological origin?
Floor of pharynx
Which form of the thyroid hormone is active and which is inactive?
Which is the prohormone?
T4
T3
Thyroglobulin - produced by follicular cells combined with iodine from blood to create T3 and T4 in presence of thyroperoxidase
T4–> T3 in presence of deiodinase
Hyperthyroidism causes
Autoimmune (Grave’s disease) or nodule- related
