Background: Beta blockers are frequently prescribed for symptomatic relief in thyrotoxicosis prior to the response to thionamides or radioiodine therapy. Propanolol has an additional mechanism of inhibiting 5’-deiodination, reducing conversion of serum T4 to T3.
Case: A 40-year-old Indigenous man from a remote community 200km west of Alice Springs, presented with transient left leg weakness and underwent investigation and management for a transient ischaemic attack. During this work up, he was concomitantly diagnosed with Graves’ disease with thyroid stimulating hormone <0.01mIU/L (0.40-4.05), elevated free T4 85.4pmol/L(8.5-27.0), elevated free T3 29.8pmol/L(4.3-8.1) and positive thyroid stimulating immunoglobulins 4.19IU/L(>0.55). Carbimazole 15mg twice daily was commenced along with propranolol 20mg twice daily for symptomatic sinus tachycardia. Three days later, a precipitous drop in sodium was noted, from baseline 136 to 122mmol/L. Serum osmolality was 262mmol/kg (275-300), urine osmolality 478mmol/kg, urine sodium 93mmol/L, morning cortisol 456nmol/L. Both carbimazole and propranolol were ceased, and a 750ml/24hour fluid restriction commenced, with subsequent normalisation of sodium over five days. He was recommenced on the same dose of carbimazole with sodium remaining stable over the next three days, 140mmol/L. His fluid restriction was lifted and he was discharged. The hyponatraemia was thus deemed to be induced by propranolol.
Conclusion: Graves’ disease is usually managed in the outpatient setting with propranolol being commonly prescribed to manage symptoms. This inpatient case demonstrates the rare adverse effect of hyponatraemia that can occur shortly after commencement of beta blockers. Case reports have linked propranolol and atenolol with increased risk of severe hyponatraemia. The mechanism is unclear but may relate to renin inhibition[1]. Serial biochemistry for thyrotoxicosis is usually performed 6 to 8 weeks following commencement of medical therapy with a primary focus on thyroid function tests. However, increased vigilance and early monitoring of electrolytes may be necessary to avoid other complications of therapy.