A 26-year-old woman comes to the office due to excessive fatigue and decreased exercise capacity for the past several weeks. She delivered a healthy boy 5 months ago with no complications and has been attributing her symptoms to "baby blues." The patient enjoys spending time with her son and is excited to return to work. She has had no change in appetite but has been gaining weight. The patient has had no insomnia, excessive urination, feelings of worthlessness, or suicidal ideations. She takes no medications and does not use alcohol, tobacco, or illicit drugs. Temperature is 36.5 C (97.7 F), blood pressure is 136/92 mm Hg, and pulse is 64/min. Weight is 70 kg (154 lb) and BMI is 27 kg/m². Physical examination shows moist mucous membranes and no scleral icterus, lymphadenopathy, or jugular venous distension. The lungs are clear on auscultation and heart sounds are normal. The abdomen is soft and nontender. There is mild, nonpitting edema of the hands and feet. Laboratory results are as follows: Which of the following is the definitive treatment for this patient's hyponatremia?

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Explanation: 11ec2b69_c096_c789_8f63_315fd588f095_MD0005_00 This patient has euvolemic hypo-osmolar hyponatremia. In addition, she has fatigue, weight gain, and a significant elevation in TSH consistent with primary hypothyroidism. In light of her recent pregnancy, this is most likely due to postpartum thyroiditis, an autoimmune disorder that is considered a variant of chronic lymphocytic (Hashimoto) thyroiditis. Moderate to severe hypothyroidism (with significant elevations in TSH) can be associated with mild hyponatremia, possibly due to decreased clearance of free water and increased release of antidiuretic hormone. Classic postpartum thyroiditis is characterized by a brief thyrotoxic phase, followed by a self-limited hypothyroid phase and eventual return to a euthyroid state. Mild hypothyroidism due to postpartum thyroiditis does not require treatment and typically resolves within a few months. However, moderate to severe hypothyroidism, as in this symptomatic patient, should be treated with levothyroxine, which will likely correct the hyponatremia. (Choice A) Restricting free water would improve this patient's hyponatremia but would not be the definitive therapy as it would not correct the underlying hypothyroidism. (Choice B) Glucocorticoids are recommended in the initial management of adrenal insufficiency. Primary adrenal insufficiency can cause hyponatremia, but this patient has no other signs of primary adrenal insufficiency (eg, hyperpigmentation, weight loss, hypotension, hyperkalemia), and her elevated TSH is consistent with hypothyroidism. (Choice C) Isotonic saline is used for treating hypovolemic hyponatremia (eg, gastrointestinal losses). Infusion of isotonic saline in euvolemic hypo-osmolar hyponatremia with an inappropriately elevated antidiuretic hormone level can worsen the hyponatremia. (Choice E) Vasopressin receptor antagonists (eg, tolvaptan) may be considered for treating severe hypervolemic or euvolemic hyponatremia that is symptomatic and resistant to fluid restriction. They are not needed for mild hyponatremia. Educational objective: Moderate to severe hypothyroidism can be associated with mild euvolemic hyponatremia. Treatment of the underlying hypothyroidism with levothyroxine can correct the hyponatremia. References: Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. (http://www.ncbi.nlm.nih.gov/pubmed/24074529) Management of endocrine disease: hypothyroidism-associated hyponatremia: mechanisms, implications and treatment. (http://www.ncbi.nlm.nih.gov/pubmed/27484454)

A 26-Year-Old Woman Comes to the Office Due to Excessive