A 14-year-old girl presented to her general practioner with neck swelling and lethargy. Initial thyroid function showed a free T4 of 6.6 pmol/L (12.6–21.0 pmol/L) and thyroid stimulating hormone 34.9 mIU/L (0.51–4.3 mIU/L). An ultrasound of the thyroid was in keeping with active thyroiditis and anti-TPO antibodies were positive indicating autoimmune hypothyroidism. She had no family history of note. She was started on levothyroxine 75 µg daily with normalisation of her thyroid function within 3 weeks.

She was admitted to the emergency department 4 weeks later with progressive fatigue and acute nausea and vomiting. At presentation, she was lethargic, tachycardic at 122 bpm, hypotensive at 95/57 mmHg and afebrile.

Initial venous blood gas: pH 7.23, HCO3 13.6 mmol/L, pCO2 4.2 kPa, BE −12.9 mmol/L, Cl 100 mmol/L, Lac 1.5 mmol/L, Glu 3 mmol/L, Na 121 mmol/L and K 4.1 mmol/L.

Initial investigations: urea 4.2 mmol/L, creatinine 43 µmol/L, adjusted calcium 2.45 mmol/L.

What is the most likely diagnosis?

Acute renal failure

Adrenal crisis

Myxoedema crisis

Sepsis …