Hypothyroidism Flashcards
What is the epidemiology of hypothyroidism?
UK prevalence:
1% women, 0.5% men
Lifetime risk – 9% women, 1% men
Autoimmune hypothyroidism/Hashimoto’s = the most common in countries with adequate iodine intake; iodine deficiency is most common worldwide
Congenital:
1/4000 babies
Screening programs as a result
What are the broad categories of hypothyroidism?
Primary Hypothyroidism:
- Often autoimmune; iodine deficiency; thyroid damage e.g. from radioiodine used to treat hyperthyroidism; drugs e.g. lithium, amiodarone; transient and post-partum thyroiditis
Central hypothyroidism (secondary and tertiary):
- TSH levels are inappropriately low or normal (though rarely raised), but free T4 is below the normal reference range
- Is rarer than primary disease
- Secondary to pituitary or tertiary hypothalamic disorders
- e.g. pituitary adenoma, pituitary apoplexy, Sheenhan syndrome, SAH, haemochromatosis
Can also be congenital
- Thyroid dysgenesis (majority) e.g. aplasia/partial genesis/ectopic tissue
- Dyshormogenesis e.g. problems with enzymes/pathways required for synthesis
- Screened for
Overt vs. subclinical:
- Overt = TSH above the normal reference range (usually >10mU/L) and free T4 (thyroxine) is below the normal reference range; often primary causes
- Subclinical hypothyroidism = TSH above normal reference range but T3 and T4 are within the normal range
What is Hashimoto’s thyroiditis?
Aetiology:
Autoimmune condition
Follicular cells attacked by T-cells (as in atrophic)
Pathophysiology:
Autoimmune
Goitre - enlarged thyroid due to infiltration with lymphocytes and subsequent fibrosis
Thyroid often becomes firm and rubbery
TPO:
Thyroid peroxidise is an enzyme that ionises iodine to I+ read for release into colloid
Without the enzyme there will not be enough I+ to make T3 and T4
The antibodies to TPO will be found in high concentrations (>1000 U/L) in the blood of affected patients
What is postpartum thyroiditis?
Seen transiently after pregnancy - between 3-8 months (mean 6m), lasting 4-6m
May be hyper/hypo or both
Hyper – as a result of beta-hCG acting on TSH receptors → production of more thyroid hormone
What is iodine deficiency?
Common in mountainous areas
May be endemic goitre:
Caused by underlying deficiency → stimulates TSH production → thyroid enlargement to compensate for low levels of iodine
What is congenital hypothyroidism?
Mostly due to failure of thyroid to develop or develop enough
Also due to ectopic tissue
I.e. thyroid tissue located in a different anatomical position
Dyshormonogenesis:
Defects in production of thyroid hormones
How does hypothyroidism present?
Most commonly presents in middle aged women – common differential diagnoses are depression and chronic fatigue syndrome
Change in appearance:
Dry, brittle, thin hair, eyebrow loss, goitre (auto immune), puffy eyes, dry skin
Deep/hoarse voice
Weight gain and poor appetite, obesity
Constipation
Menorrhagia or oligo/amenorrhoea
Cold intolerance, hypothermia
Poor memory/mental slowness, depression, poor libido
Malaise, lethargy
Peripheral oedema including eyelid swelling
Bradycardia, diastolic hypotension
MOM SO TIRED: Memory loss Obesity Malar flush/ Menorrhagia Slowness (mentally and physically) Skin and hair dryness Onset gradual Tiredness Intolerance to cold Raised BP Energy levels fall Depression/ Delayed relaxation of reflexes
How does hypothyroidism present in the elderly? Children? Neonates?
Elderly – lots of signs are similar to normal ageing process – hard to differentiate e.g. fatigue, cold intolerance, weight gain, constipation, depression
Children – rare
Retarded growth, infantile face, delayed puberty
Neonates – very rare
Failure to thrive, prolonged jaundice, feeding difficulties, constipation
What are some complications of hypothyroidism?
Metabolic syndrome (central obesity, dyslipidaemia, hypertension and diabetes) (leading to)
Coronary heart disease, stroke, heart failure
Neurological and cognitive impairments
Adverse maternal and foetal outcomes in pregnancy
Myxoedema:
Skin and tissue disorder associated with prolonged hypothyroidism
Thickening of skin and subcutaneous tissues
Caused by accumulation of mucopolysaccharide
Myxoedema coma:
All usual features plus coma and possible seizures
Often occurring in patients with multiple morbidities i.e. heart failure, stroke etc and who are undiagnosed
Mortality is 50% - emergency admission is necessary
Pulmonary and cardiovascular support + IV T4
How do you investigate hypothyroidism?
Hx and Ex.
TSH test:
0.4-4U/L
Needs to be consistently above 10U/L to be able to diagnose hypothyroidism
This test alone is enough to diagnose primary hypothyroidism
Free T4 test:
9-25pmol/L (1% of total T4, rest is bound to thyroid binding globulin)
Usually performed in conjunction with the TSH test as it allows to confirm a hypothyroid state
Low level of T4 will confirm hypothyroid state
T4 half-life of 1wk - therefore to monitor impact of increasing levothyroxine, need to wait several weeks
If TSH normal/low and T4 also low = TSH deficiency, NOT hypothyroidism as low T4 would feedback to hypothalamus to stimulate more TSH; thus this test is also used to check for secondary hypothyroidism
T3 level measurement:
3.5-7.8nmol/L
NOT used for Dx as T4:T3 released stands at 20:1 with most of the T3 being converted from T4 peripherally so in a proportion of patients, T3 levels will be normal
Anaemia:
A common finding on doing a FBC
Often normocytic
Can be macrocytic if pernicious anaemia present
Can be microcytic if menorrhagia present
TPO antibodies:
Can be checked to confirm auto-immune hypothyroidism
What drug treatment do you use for hypothyroidism?
In young patients w/o heart disease:
Levothyroxine (LT4)
Treatment for life
Review symptoms and TFTs every 3/12 and make LT4 adjustments as necessary
Older patients, possibly with heart disease or other complications:
Same as above but start at lower doses and gradually increase
In those with no functioning thyroid:
150micrograms daily
Other risks of overtreatment: atrial fibrillation (c. 10% of those over treated), anxiety, sweating, insomnia, other arrhythmias
Pregnancy:
Increase dosage as babies born when mothers TSH levels are high tend to have impaired cognitive function
What advise do you give those newly diagnosed with hypothyroidism?
Risk of other disorders developing:
Addison’s disease
Pernicious anaemia
Depression, potentially a severe (but transient) psychosis
When should you refer a woman with overt or subclinical hypothyroidism?
If they are planning a pregnancy or are pregnant (for the latter - urgent TFTs needed and LT4 dose adjustment as per specialist advice)
Postpartum and was treated with LT4 in pregnancy
Diagnosis of postpartum thyroiditis
