Hypothyroidism Flashcards
What is hypothyroidism?
Hypothyroidism aka myxoedema = underactive thyroid and as a result lack of thyroid hormone
What is the function of the thyroid?
- Increases heart rate & cardiac output
- Increases bone turnover & resorption
- Increases gut motility
- Increases speed of muscle contractility/relaxation and muscle protein turnover
- Increases hepatic gluconeogenesis/glycolysis & intestinal glucose turnover
- Increases beta adrenergic receptor numbers in heart, skeletal muscle and adipose cells + decreases cardiac alpha adrenergic receptors (sympathetic activity)
What is the feedback mechanism of the thyroid axis?
Thyrotrophin releasing hormone (TRH) produced in hypothalamus
=> TRH stimulates pituitary to secrete thyroid stimulating (TSH)
=> TSH stimulates growth of thyroid follicular cells via g-protein coupled TSH receptor
=> T3 & T4 released into circulation
=> Circulating T4 peripherally deiodainated to T3 (active form)
=> T3 binds to thyroid hormone nuclear receptor on target organ cells => modified gene transcription
What is the role of iodine in thyroid hormone production?
Iodine is oxidised and incorporated into thyroglobulin to form T3 and T4
Without iodine - thyroid hormone cannot be produced
What are the causes of hypothyroidism?
Primary causes:
- Primary atrophic hypothyroidism (women>men)
- Hashimoto’s thyroiditis
- Iodine deficiency (worldwide main cause of hypothyroidism)
- Drug induced i.e. Amiodarone, lithium, iodine, anti-thyroid drugs
- Subacute hyperthyroidism - temporary hypothyroidism after hyperthyroid phase
- Post thyroidectomy or radio iodine treatment
- Congenital
Secondary causes:
- Hypopituitarism - not enough TSH production
Who does hypothyroidism commonly affect?
Women > men 6:1
Both primary atrophic hypothyroidism and Hashimoto’s thyroiditis are autoimmune conditions.
What is the main difference between the two?
Primary atrophic hypothyroidism :
i) Anti-thyroid antibody => diffuse lymphocytic infiltration of the gland => atrophy and fibrosis => no goitre
ii) Most common cause of hypothyroidism
iii) Women > men 6:1
Hashimoto’s thyroiditis:
i) Thyroid peroxidase anti-body => lymphocytic & plasma cell infiltration => atrophic changes with regeneration => goitre formation
ii) Women > men
iii) Most common in late middle ages
iv) Patient can be hypothyroid or euthyroid
What are the symptoms of hypothyroidism?
Tiredness ; sleepy ; lethargic
Low mood
Cold disliking
Increased weight
Constipation
Menorrhagia
Hoarse voice
Poor memory / cognition / dementia
Myalgia / cramps / weakness
Poor libido
What are the signs of hypothyroidism?
BRADYCARDIC acronym
BRADYCARDIC
B = Bradycardia*
R = Reflexes relax slowly*
A = Ataxia (cerebellar)
D = Dry, thin hair + skin*
Y = Yawning / drowsy / coma
C = Cold hands ± hypothermia
A = ascites ± non-pitting oedema ± pericardial/pleural effusion
R = Round puffy face, obese
D = Depression
I = Immobile ± ileus
C = congestive cardiac failure ± myopathy ± neuropathy ± goitre
*most differentiating signs
Why does iodine deficiency cause hypothyroidism?
Iodine deficiency = no T3/T4 made
=> Increased TSH production
=> Thyroid enlargement
WATCH OUT FOR:
Atypical presentations
- Children with hypothyroidism = slow growth velocity, poor school performance
- Young women with hypothyroidism
=> hypothyroidism should be excluded in all women with oligomenorrhoea/amenorrhoea, menorrhagia, infertility or hyperprolactinaemia
INFO CARD
How do you diagnose hypothyroidism?
Thyroid function test:
Serum TSH => investigation of choice
=> high TSH level confirms primary hypothyroidism
=> low free T4 level confirms hypothyroid state
Cholesterol & triglycerides high too
Macrocytosis
Normocytic normochromic anaemia
Treat patient not the blood level
The aim is to restore T4 and TSH within normal range.
How do you treat hypothyroidism?
Healthy & young : Levothyroxine 100ug daily
=> Assess at least 6 weeks
=> Once TSH normal, review annually via thyroid function test
Elderly or ischaemic heart disease patient : 25ug Levothyroxine
=> Assess every 4 weeks to increase dose by 25ug according to TSH level
Caution!! Levothyroxine may precipitate angina or MI
=> Full resolution of symptoms ~6 months
What is myxoedema coma?
Severe hypothyroidism that presents with confusion or even coma esp in elderly
=> very rare
=> can present with hypothermia, severe cardiac failure, pericardial effusions, hypoventilation, hypoglycaemia, hyponatraemia
=> Severe hypothyroidism can lead to psychosis
What are the hypothyroid associations?
Autoimmune Hypothyroidism seen with other autoimmune diseases i.e. Type 1 DM, Addison’s, pernicious anaemia
Hypothyroidism also assoc. with Turner’s, Down’s, cystic fibrosis, primary biliary cholangitis, polyneuropathy, organomegaly
Genetics: Dyshormonogenesis - defect in hormone syntesis
