Life-threatening hyponatremia due to cessation of L-thyroxine

Electrolyte disorders in hypothyroidism are frequently subtle and rarely observed in clinical practice. A 50-year-old woman was admitted to the emergency room with complaints of nausea, weakness, and lethargy. Her medical history revealed total thyroidectomy two years earlier. She was commenced on L-thyroxine after the surgery. However, the patient stopped the treatment for three months. Thyroid function tests showed free T3 0.80 pg/ml (n: 1.8-4.2), free T4 <0.20 ng/dl (n: 0.8-1.9), TSH 56.84 microU/ml (n: 0.4-4.0). Her biochemical and laboratory investigations were normal, except for a plasma sodium value of 114 mmol/L (n: 135-145). Hypertonic saline treatment with L-thyroxine was immediately started. Symptomatic hyponatremia caused by hypothyroidism was the direct consequence of cessation of L-thyroxine treatment. The patient was followed up for a year and still using L-thyroxine (0.1 mg). In conclusion, it should be kept in mind that life-threatening hyponatremia may occur in patients with total thyroidectomy-induced hypothyroidism; L-thyroxine should be immediately started if stopped for any reason.