Week 3- The Thyroid Gland Flashcards
How many lobes does the thyroid have?
Usually 2, some people have 3 (10-15% of people have a third lobe called the pyramid)
What is the gap that joins the 2 lobes of the thyroid?
Isthmus
What is the thyroid comprised of?
Follicles
What glands are embedded in the thyroid?
Parathyroid
What nerve associated with the thyroid supplies the vocal chords?
Left recurrent laryngeal nerve
What are the 2 glands in the thyroid that are important for calcium metabolism?
Inferior and superior parathyroid glands
What part of the cell are the thyroid hormones made?
In the colloid
What are the 2 thyroid hormones called?
T3 (triiodothyronine) and T4 (thyroxine)
Out of T3 and T4, which is active and which is inactive
T3 is active
T4 is inactive
What 2 molecules couple to form T3?
Monoiodotyrosine and diiodotyrosine
What 2 molecules couple to form T4?
2 molecules of triiodothyronine
What is the main hormone product of the thyroid gland?
T4
How is T4 activated?
It is deiodinated to T3 in target tissues
What is reverse T3?
A molecule produced when T4 is deiodonated in a different position to produce an inactive molecule
What enzyme converts T3 to T4?
Deiodinase enzyme
How do thyroid hormones affect cells?
They bind to receptors and change gene expression
Why is the mothers thyroid hormone status important in pregnancy?
The foetus gets its thyroid hormone from the mother so essential for growth and development
What is cretinism?
Untreated congenital hypothyroidism
How is congenital hypothyroidism measured in babies?
Heel prick test
What are some of the main actions of the thyroid hormone?
It increases basal metabolic rate, increases cardiac output, affects GI, CNS, reproductive system
What is the half life of T3
2 days
What is the half life of T4?
7-9 days
A lot of which molecule inhibits T3/T4 production?
Iodide
Are thyroid disorders more common in women or men, by how much and why?
In women by a 4:1, due to autoimmunity
Are over or under active thyroids more common?
Neither they’re the same
What happens in hypothyroidism and how does it arise?
Thyroxine levels decline and TSH levels climb, due to autoimmune damage
What are the 2 main common thyroid disease?
Hashimoto’s thyroiditis (hypothyroidism)
Graves’ disease (hyperthyroidism)
What are common symptoms of hypothyroidism?
Fatigue Depression Deep voice Cold intolerance Weight gain Reduced appetite Constipation Bracycardia Eventual myxedema coma
What medicine is given to treat hypothyroidism?
Levothyroxine
It can also treat hyperthyroidism sometimes
What is the common dose of levothyroxine?How is it administered?
100 micrograms
Administered orally
What is common thyroid hormone replacement?
Giving a mix of T3 as well as T4
What happens in hyperthyroidism?
Thyroxine levels rise
TSH levels drop
What are the 3 main causes of hyperthyroidism?
Graves’ disease (whole gland smoothly enlarged and whole gland overactive)
Toxic multinodular goitre
Solitary toxic nodule
What is the mechanism of action in Graves’ disease?
Antibodies bind to and stimulate TSH receptors in the thyroid causing a smooth goitre
What happens in Graves’ disease when antibodies bind to muscles behind the eye?
Exophthalmos- the eyes bulge and you can see the whites around the eye
What happens in Graves’ disease when antibodies bind to soft growth tissue of shins?
Pretibial myxoedema
What are common symptoms of hyperthyroidism?
Heat intolerance Weight loss with increased appetite Myopathy Mood swings Diarrhea Tremor of hands Palpitations Sore eyes Goitre
What 2 cell types are present in thyroid follicles?
Follicular cells and colloid cells
Describe the embryology of the thyroid gland
Originates from the base of the tongue (floor of the pharynx)
Development of thyroglossal duct
Divides into 2 lobes
Duct disappears leaving the foramen cæcum
Thyroid gland then develops
Where does circulating T3 come from?
80% from deiodination of T4
20% from direct thyroidal secretion
How are thyroid hormones transported in the blood?
They are bound to plasma proteins (99% of the time)
What are the 3 main plasma proteins that transport thyroid hormones?
Thyroid binding globulin
Albumin
Prealbumin/ transthyretin
How is thyroid hormone production controlled?
The negative feedback loop prevents excess production as T3/T4 production switches off TRH in hypothalamus (indirectly via switching off TSH) and TSH production in the anterior pituitary (directly)
What are potential minor complications of taking levothyroxine?
Weight loss and headache
What are potential major complications of taking levothyroxine?
Heart attack and rapid heart rate
What is thyroglobulin, where is it produced and what special property does it have?
It is the prohormone produced in the anterior pituitary gland that has a thymine residue that can be iodised. It allows the synthesis of T3 and T4 in the presence of thyroperoxidase
Describe the pathway for T3/T4 synthesis in the thyroid gland
TSH is produced by the anterior pituitary gland and binds to TSH receptors on follicular cells
Iodide is transported into the follicular cell via sodium iodine receptor and then to the colloid
Iodide is oxidized to iodine
TSH binding also starts production of thyroglobulin (a proenzyme that has a thymine residue that can be iodised)
Thyroperoxidase enzyme is also activated by TSH binding and travels to the colloid where it catalyses iodination reactions
Iodination of thyroglobulin allows synthesis of T3 and T4 (that are still bound to thyroglobulin)
Protein bound T3 and T4 travel back to follicular cells where peptide bonds are broken and T3/T4 are released into the bloodstream
What molecule is essential in the synthesis pathway of T3/T4?
Iodide (but it must be oxidized to iodine in the synthesis pathway)
How does TRH from the hypothalamus reach the anterior pituitary?
It is released into the medial leminence and as the vessels are leaky travels via portal circulation
What does ‘primary’ in primary hypothyroidism mean?
That it is a problem with the gland
What is menorrhagia and what thyroid condition is it a common feature of?
Heavy peroids- a common feature of hypothyroidism
How much time after initial levothyroxine prescription is the dosage checked and how is it checked?
Checked after 6/8 weeks
If the dose is too high TSH levels will be too low
If the dose is too low TSH levels will be too high
