Lecture 22 Flashcards
When is the thyroid gland visible/palpable?
When enlarged (here it is called a GOITRE)
What is the clinical significance of a goitre?
Can compress structures causing issues with swallowing and breathing
-could be cancer
What nerves are likely to be damaged during thyroid surgery?
Recurrent laryngeal
External branch of the superior laryngeal
(these supply the larynx which is involved in speech)
What blood vessels supply the thyroid?
3 arteries and 3 veins
-superior, middle and inferior thyroid arteries/veins
It is a highly vascularised structure
What is the most common cause of a giotre?
Iodine deficiency
Why do you take iodine in pregnancy?
To ensure you don’t develop hypothyroidism which can lead to cretinism in children
What is the size and weight of the thyroid?
2-3cm
Weighs 15-20g making it one of the largest endocrine glands in the body
What are the 2 major cell types in the thyroid?
Follicular cells: arranged in follicles Parafollicular cells (C cells): found in connective tissue separating the follicles
What are the 3 hormones produced by the thyroid gland?
Thyroxine: T4
Triiodothyronine: T3
Calcitonin (via parafollicular cells)
What amino acid are T3/4 derived from?
Tyrosine (tyrosine residues on thyroglobulin add iodine)
How long does the store of T3/4 last in the lumen of the follicles?
Several months
2-3 months
Once T3/4 are cleaved from thyroglobulin how do they reach target tissues?
They diffuse from the epithelial cells into circulation
Where is TRH released from?
Dorsomedial nucleus of the hypothalamus under the influence of circulating levels of T3/4 (negative feedback), stress, and temperature
What is TSH stimulated by and where from?
Stimulated by TRH from hypothalamus. TSH is released from thyrotropes in the anterior pituitary. TSH travels in the blood to affect the follicular cells of the thyroid gland
What rhythm does TSH release follow?
Diurnal rhythm, higher levels at night decreasing in the early hours of the morning
What trophic effects does TSH have?
-increased vascularity
-increase in size and number of follicle cells
Can result in a goitre
Are T3/4 hydrophobic/hydrophilic?
Hydrophobic
Does T3/4 have a lower affinity for transport proteins?
T3, therefore there is a higher % of free T3 and its half life is therefore shorter
What increases the synthesis of TBG (thyroxine binding protein)?
Oestrogen during pregnacy, producing a fall in the amount of free T3/4as more is bound, therefore more TSH/TRH are produced and the thyroid gland secretes more T3/4
(therefore the amount of free T3/4 returns to normal but the total amount in the blood is increased)
What specific effects do T3/4 have on certain tissues?
- directly affect bone mineralisation (physical growth)
- increase synthesis of heart muscle protein
- CNS: hyperplasia of cortical neurones (in cerebral cortex) and myelination
What are some indirect actions of T3/4?
Stimulate hormone and neurotransmitter receptor synthesis in variety of tissues (heart/GI) producing an increased responsiveness
-tachycardia
-increased bowel motility
Permissive role in actions of FSH and LH, therefore ovulation fails to occur in absence of TH (period)
Do target receptors have a greater affinity for T3/4?
T3
What does increased protein synthesis due to activation of transcription factors by T3/4 mean?
Increased production of specific functional proteins, leading to increased cell activity and increased demand in energy
How is T4 converted to T3?
Removal of the 5 prime iodine
What does the removal of the 3 prime iodine on T3 cause?
Produces rT3, an inactive reverse which can bind to TH receptors without stimulating them, blocking the effect of T3
What is Hashimoto’s disease?
Autoimmune disease mostly affecting women, resulting in destruction of the thyroid follicles/ production of an antibody which blocks the TSH receptor on follicle cells preventing them from responding to TSH
=Hypothyroidism
-low T3/4
-high TSH/TRH
How are patients with Hashimoto’s disease treated?
With oral thyroxine- levothyroxine (thyroxine replacement therapy)
What is Grave’s disease?
Form of hyperthyroidism mostly in women, autoimmune disease where antibodies (thyroid stimulating immunoglobulin-TSI) stimualte TSH receptors on follicle cells resulting in more T3/4.
-high T3/4
-low TSH/TRH
TSH levels fall due to negative feedback but doesn’t effect TH as TH secretion is due to the TSI rather than TSH
How do you treat Grave’s disease?
Carbimazole: inhibits incoorperation of iodine into thyroglobulin by inhibiting thyroid peroxidase
-beta blockers can be used to dampen sympathetic system: can cause agranulocytosis: neutropenia: risk of infections
Symptoms of hypothyroidism:
- cold intolerance
- reduced BMR
- weight gain
- tiredness
- bradycardia
- neuromuscular system (weakness, cramps, cerebellar ataxia: clumsiness of movement)
- dry, flaky skin
- alopecia
- voice is deep
- mixed oedema
- goitre
- bulging eyes (as autoimmune antibodies attack back of eyes)
Symptoms of hyperthyroidism:
- heat intolerance
- increased oxygen consumption
- increased BMR
- weight loss
- physical and mental hyperactivity
- tachycardia
- intestinal hypermobility
- skeletal and cardiac myopathy
- osteoporosis due to increased bone turnover
Does the thyroid gland move?
Yes it moves upon swallowing
Where would you find a thyroglossal cyst?
In the midline of the throat
What is borderline hypothyroidism?
Normal T4/3 but high TSH
Symptoms of Hashimoto’s disease?
- loss of lateral 1/3 of eyebrow
- carpel tunnel syndrome due to fluid retention, pressure on median nerve
What is myxeoedema?
Swelling around eyes and skin
Symptoms of Grave’s disease?
- pretibial myxoedema (autoantibodies affect skin in shin)
- clubbing of nail beds
What damage can removal of thyroid gland surgically have?
Damage to parathyroid glands and therefore calcium homeostasis
When does a nodule in the thyroid appear cancerous?
- single nodule
- rapid enlargement of lump in neck
- family history of thyroid cancer
- hard/irregular feeling
- lymphnodes present
How could you determine if a thyroid lump was cancerious or not?
Do a biopsy with ultrasound. Take cells via biopsy to investigate them
-if cancerous you can surgically remove
How do you image the thyroid?
Ultrasound
X ray
CT
What is an ethical issue with thyroid drugs?
They can be abused
What is lingual thyroid?
Thyroid tissue in tongue, didn’t migrate fully
What is myxoedema coma?
Hypothermia and fluid overload in heart: severe hypothyroidism in elderly
What is thyroid crisis/storm?
Hyperpyrexia, tachycardia, cardiac failure
=urgent treatment (hyperthyroidism)
Why do you have to be careful if pregnant with Grave’s disease?
Antibodies can cross the placenta and baby can be born with hyperthyroidism
