A 15-year-old girl, previously well, presented to her family physician with a 2-month history of fatigue and exercise intolerance. There was no history of fever, syncope, palpitations, wheezing, polyuria, polydipsia, constipation or cold intolerance. She had no sick contacts. Breast development began at 12 years but she had not experienced menarche. There were no medications or recreational drug use. Her father has type 1 diabetes mellitus and her maternal grandfather had acquired anemia. Investigations included elevated thyroid stimulating hormone (TSH) and low free T4. She was diagnosed with hypothyroidism. Levothyroxine was initiated at 50 μg (1.3 μg/kg) daily, thyroid function tests were repeated 6 weeks later without dose change. Over the ensuing 3 months, there was no symptomatic relief. Rather, she reported progressive fatigue, abdominal pain and 5 lbs weight loss. Three months later, she presented to our emergency department. Blood pressure was 92/60, heart rate was 90 bpm without orthostatic changes. Cardiovascular, respiratory, abdominal and neurologic exams were normal. TSH was 14 mIU/L (0.5 to 5.0); free T4 was 11.3 pmol/L (10.0 to 23.0). Complete blood count (CBC), C-reactive protein (CRP), glucose, electrolytes and urinalysis were normal. Levothyroxine was increased to 75 µg daily. Ten days later, she returned to the emergency department with hypotension and worsening fatigue. She was afebrile. Blood pressure was 72/44 without orthostatic change. Supine heart rate was 118 bpm and standing was 140 bpm. Thyroid examination was normal. Examination and further bloodwork confirmed the diagnosis.