The thyroid gland (12) Flashcards
Where is the thyroid located?
in neck, around trachea, below thyroid cartilage
N.B. superior and inferior parathyroid glands embedded in thyroid
and recurrent laryngeal nerve runs close- supplies vocal cords
How can we describe the structure of the thyroid gland?
isthmus separates the 2 lobes
roughly 10-30% people have extra lobe - pyramidal lobe- embryological variant
How does the thyroid develop? (embryology)
- originates from base of tongue
- thyroglossal duct descends
- divides into 2 lobes
- duct disappears, leaving foramen caecum
- final position of thyroid gland at week 7 of gestation, then develops
How can we describe the histology of the thyroid gland?
follicles in thyroid- made up of follicular cells which surround spheres/sticky colloid (where thyroid hormone made)
also parafollicular cells- secrete calcitonin
How is thyroid hormone made?
- TSH secreted from anterior pituitary, arrives via bloodstream and binds TSH-R on follicular cell–> triggers production of thyroglobulin TG (a prohormone with tyrosine residues) AND activates enzyme TPO (thyroperoxidase)–> travels into colloid + catalyses iodination reactions
- IODIDE ions also arrive- transported into follicular cell–> then enters colloid
- iodide oxidised to iodine
- iodine sticks onto tyrosine residues of TG–> forms MIT and DIT–> couple to form T3 (active) and T4
- T3 and T4 move back into follicular cell- protein bonds broken down to release from TG–> then enter systemic circulation
What are MIT and DIT and what happens when you combine them?
monoiodotyrosine and diiodotyrosine (iodine/2 iodine been added to tyrosine)
if you combine MIT and DIT–> triiodothyronine (T3)
if you combine DIT and DIT–> tetraiodothyronine/thyroxine (T4)
What is reverse T3?
inactive hormone
- T4 has been deiodinated in a different position
Where do the proportions of circulating T3 come from?
N.B. a healthy adult thyroid gland secretes T3 AND T4
T4= prohormone- converted into more active T3 by deiodinase enzyme in peripheral tissues
! 80% of T3 comes from deionisation of T4 !
! 20% from direct thyroidal secretion !
How does thyroid hormone travel around the body?
- mostly bound to plasma proteins e.g. thyroid-binding globulin (TBG), albumin, and prealbumin
- less than 1% thyroid hormone is unbound, so very small amount actually active
How does thyroid hormone affect gene transcription?
T3 and T4 enter target cell via receptors–> T4 activated by deiodinases- becomes T3–> enters nucleus and binds to thyroid hormone receptor in nucleus
Why do we need thyroid hormone?
- essential for fetal growth and development, esp. CNS
(N.B. untreated congenital hypothyroidism= cretinism)
(heel prick test measures TSH in babies) - increases basal metabolic rate
- protein, carbohydrate and fat metabolism
- effects on GI, CNS, reproductive systems
- affect sympathetic nervous system by potentiating actions of catecholamines (e.g. tachycardia, lipolysis)
What controls thyroid hormone production?
- hypothalamus produces TRH–> enters anterior pituitary–> stimulates thyrotrophs to release TSH (thyrotropin)–> TSH enters systemic circulation–> arrives at thyroid–> T3 and T4 production
- T3 and T4 negative feedback to anterior pituitary and hypothalamus- ‘hypothalamo-pituitary thyroid axis’
- somatostatin inhibits TSH production
- iodide inhibits hormone production
Are thyroid disorders more common in men or women?
women (4:1 ratio) bc more at risk for autoimmune disorders
Is an hypothyroidism or hyperthyroidism more common?
equally prevalent
What is the main cause of primary hypothyroidism?
autoimmune damage to thyroid–> thyroxine levels dec. –> TSH levels inc. (bc wants to make T4)
also from surgery (if thyroid removed)
What are the commonest forms of autoimmune thyroid disease?
Hashimoto’s thyroiditis (hypo) and Graves’ disease (hyper)
N.B. presence of 1 autoimmune disease inc. risk of others
What are the symptoms and signs of hypothyroidism?
- deepening voice
- depression and tiredness
- intolerance to cold
- weight gain w/ reduced appetite
- constipation
- bradycardia
- eventual myxoedema coma
How do we treat hypothyroidism?
tablet- levothyroxine (same as T4)… sometimes used for hyperthyroidism (blocking+replacement regimen)
N.B. don’t need T3 as deiodinase enzymes in whole body
What is the common dosage of levothyroxine?
- adjusted according to TSH (to get into normal range)
- mostly administered orally (sometimes intravenously if emergency- give T3)
- common dose 50-100 micrograms
What are potential complications of levothyroxine?
if given too much- minor: can cause weight loss and headache
major: heart attack/ rapid HR
What happens in hyperthyroidism?
thyroid makes too much thyroxine–> thyroxine levels rise–> TSH levels drop
What are the causes of hyperthyroidism?
- Graves’ disease- autoimmune- thyroid gland enlarged–> overactive
- nodules that release too much thyroxine (multi or singular)
What is Graves’ disease?
autoimmune disease
- antibodies bind to and stimulate TSH receptor in thyroid (should normally just respond to TSH)
- -> disrupts negative feedback loop
- causes smooth goitre (enlargement of gland)
- other antibodies bind to muscles behind eye (exophthalmos)
- other antibodies stimulate growth of soft tissue of shins (pretibial myxoedema)
What are the symptoms and signs of hyperthyroidism?
- really hot
- weight loss w/ inc. appetite
- diarrhoea
- palpitations
- mood swings
- myopathy (muscle weakness)
- sore eyes, goitre
