An Unusual Presentation of Severe Hypothyroidism in a Child With Undiagnosed Systemic Lupus Erythematous.

Myxedema is increasingly rare in the United Kingdom, particularly among patients already on treatment. We present a case of a 13-year-old girl diagnosed with autoimmune hypothyroidism who was started on 75 µg of levothyroxine by her general practitioner. She was referred to the endocrine clinic after 6 months of treatment for hypothyroidism with 2 months of worsening fatigue, chest pain, peripheral edema, and dyspnea. On hospitalization, the patient's laboratory tests revealed undetectable free thyroxine level (fT4) and elevated thyrotropin (TSH) greater than 500 μIU/mL (SI: >500 m mIU/L) (reference range, 0.51-4.30 μIU/mL [SI: 0.51-4.30 mIU/L]). She was diagnosed with thyroid myxedema and commenced intravenous (IV) hydrocortisone, IV maintenance fluids, and antibiotics. However, the constellation of signs and symptoms remained perplexing. Further tests revealed a large pericardial effusion, deep vein thrombosis, and nephritis. A multidisciplinary approach confirmed the diagnosis of systemic lupus erythematous (SLE), and the patient was successfully started on rituximab therapy within 5 days of admission. This case illustrates the importance of considering potential underlying diagnoses when assessing patients with hypothyroid crisis, especially when symptoms deviate from the typical clinical presentation. The timely recognition and management of this patient's underlying SLE were crucial in her recovery.