Hypothyroidism

Primary hypothyroidism

• Autoimmune (Hashimoto’s)
• Thyroiditis (silent, subacute, postpartum)
• Iodine deficiency
• Post-ablation (surgical, radiation, radioiodine)
• Infiltrative (lymphoma, sarcoid, amyloidosis, TB)
• Congenital
• Drugs (Amiodarone, lithium, potassium perchlorate, iodine)
• Idiopathic

Secondary hypothyroidism
 – TSH or TRH deficiency

• Panhypopituitarism
• Pituitary adenoma
• Infiltrative (haemochromatosis, sarcoidosis)
• Tumors on hypothalamus
• Brain irradiation
• Infection (TB)

Clinical features of hypothyroidism

• Symptoms
  • Hair loss, fatigue, weight gain, depression, SOB, constipation, menstrual changes, infertility, muscle cramps, joint pain, cold intolerance
• Signs
  • Periorbital puffiness, loss of outer third of eyebrow, pallor, macroglossia, hoarse voice, bradycardia, hypoventilation, reduced bowel sounds, non-pitting oedema (myxoedema), bradyreflexia, peripheral neuropathy, cool skin, rough/dry skin, hypothermia
• Subclinical hypothyroidism may increase TRH levels and result in dysfunctional uterine bleeding

Myxoedema crisis

• Multiorgan and metabolic dysfunction from severe untreated hypothyroidism
• Heralded by mental status changes, hypotension and hypothermia
• 90% of cases in elderly women during the winter
• Diagnosis is clinical and treatment should not be delayed until TFT’s back

Clinical features of myxoedema coma

• Hypothyroid signs as above +
• Bradycardia, hypotension, hypothermia, hypoventilation, ALOC and/or coma
• 50% present in SBP <100
• Infection may be precipitant without fever, leukocytosis, tachycardia or sweating
• Pleural effusions are common
• Upper airway obstruction from glottic oedema, vocal cord oedema and macroglossia
• Metabolism of sedatives and analgesics is impaired and they can accumulate to contribute to ALOC
• 75% hypothermic
  • 25% of patients are normothermic and this should suggest infection
• Hypothyroid habitus, absence of shivering and bradyreflexia can help distinguish from accidental hypothermia

Differential diagnosis

• DDx
  • Accidental hypothermia
  • Sepsis
  • Depression
  • Adrenal crisis
  • CCF
  • Hypoglycaemia
  • CVA
  • Drug overdose
  • Meningitis

Precipitants of myxoedema crisis

• Infection
• Anaesthetic agents
• Cold exposure (hence elderly women in winter)
• MI
• CCF
• Trauma
• CVA
• Gi haemorrhage
• Hyponatraemia/hypoglycaemia
• Surgery
• Burns
• Medications e.g. beta-blockers, sedatives, narcotics, amiodarone or thyroid medication non-compliance

Investigation

• Primary hypothyroidism – High TSH, low T3/T4
• Secondary hypothyroidism – Low TSH, low T3/T4
• Subclinical hypothyroidism – Mildly high TSH, normal T3/T4
• Other investigations
  • Investigate for precipitants
  • If associated with pernicious anaemia – macrocytic anaemia evident
  • If menorrhagia has occurred – microcytic anaemia
  • Hyponatraemia is common due to SIADH
  • Hypoglycaemia is common due to concomitant adrenal insufficiency or GH deficiency
  • ABG will often show hypercapnoea and respiratory acidosis
  • ECG – Low voltage bradycardia

Treatment

• Supportive care
  • ABC
  • IV dextrose for hypoglycaemia
  • Treat hyponatraemia as required
  • Vasopressors if required
  • Passive rewarming
    • Active rewarming risks vasodilatory worsening of shock state
  • Hydrocortisone 100-200mg IV (before T3/4 provided)
    • Often have associated adrenal insufficiency
• Thyroid hormone replacement
  • IV T4 4mcg/kg stat (use this in the elderly as less risk of arrhythmia/MI)
  • IV T3 20mcg stat (10mcg in elderly) then 10mcg q8h if in myxoedema coma
• Identification and treatment of precipitants

• Why T3 in myxoedema coma?
  • More rapid onset of action
  • Does not need to be peripherally converted to take action (unlike T4)
  • Cross BBB more easily than T4
  • Disadvantage is that it risks arrhythmias due to rapid onset of action
  • CI in elderly due to risk of arrhythmia/MI

Prognosis

• Myxoedema coma has high mortality rate of 30-60%
• Poor prognostic factors include bradycardia, advanced age and persistent hypotension
• All patients require ICU

Hypothyroidism in Pregnancy

• Overt hypothyroidism seen in 1-2% of pregnant women
• Subclinical hypothyroidism seen in 2.5%
• Increased requirement of thyroid hormone in pregnancy due to increased metabolic rate in mother and transplacental shift for foetal development
• Increase T4 dosing by 30% if prenatal hypothyroidism already known
• Treat subclinical hypothyroidism in pregnancy

Last Updated on October 6, 2021 by Andrew Crofton