Hypocalcemia following Treatment for Hyperthyroidism

A 17-year-old female was referred to the endocrinology clinic after blood test results suggestive of hyperthyroidism. She had mild symptoms of thyrotoxicosis, including menstrual disturbance with intermittent palpitations and tremor. On examination, the patient was normotensive, tachycardic (100 beats/min), and of slim build with poor dentition. She had a small diffuse goiter without retrosternal extension or bruit. There was conjunctival injection but no evidence of lid lag or proptosis. Auscultation of the precordium revealed murmurs in systole and diastole consistent with mixed aortic valve disease.The only child of healthy nonconsanguineous parents, the patient had previously been well. Her medical history included mild learning difficulties, a bicuspid aortic valve, recurrent urinary tract infections, and severe constipation as a child that required a colostomy, which was later reversed. Apart from an osmotic laxative, she received no other regular medication. A recent echocardiogram had demonstrated a bicuspid aortic valve with good flow and minor regurgitation.Biochemically, the patient had an undetectable serum concentration of thyroid-stimulating hormone (TSH)4 (<0.03 mIU/L; reference interval, 0.3–5.6 mIU/L) and an increased concentration of free thyroid hormone (fT4) [43 pmol/L (3.3 ng/dL); reference interval, 7.5–21.1 pmol/L]. Her baseline serum concentrations of total calcium [2.27 mmol/L (9.08 mg/dL)] and phosphate [1.26 mmol/L (3.9 mg/dL)] were both within their reference intervals (2.20–2.60 mmol/L and 0.75–1.36 mmol/L, respectively). The serum albumin concentration was 41 g/L (reference interval, 35–50 g/L), and the magnesium concentration was 0.71 mmol/L (reference interval, 0.74–1.00 mmol/L). The results of her other biochemical tests were unremarkable. An immunologic analysis demonstrated increased thyroid peroxidase antibodies (582 IU/L; reference interval, 0–60 IU/L) and increased TSH receptor antibodies (6.9 U/L; reference interval, 0–1.5 U/L), confirming Graves disease. Thyroid imaging revealed a diffusely enlarged thyroid gland, with no visible parathyroid tissue apparent on ultrasound and MRI evaluations. QUESTIONS TO CONSIDER1. What effect does thyrotoxicosis …