Background: Hyponatraemia is one of most common electrolyte abnormalities in acutely ill elderly patients. Severe hypothyroidism is often associated with hyponatraemia. The mechanism of hyponatraemia in hypothyroidism is not fully understood. It is suggested that hypothyroidism induces hyponatraemia either by inappropriate release of ADH or by decrease in GFR. Regardless of the mechanism, the net effect is impairment of water excretion. We report a case of severe hypothyroidism causing profound hyponatraemia by inappropriate secretion of ADH.
Case report: A 79-year-old lady presented with a 2-week history of confusion and lethargy. Systemic examination was unremarkable with no signs of dehydration. Routine blood investigations showed she was hyponatraemic with Na 118 mmol/l, K 4.8 mmol/l, Urea 9.2 mmol/l and Creatinine 130 umol/l. Plasma Osmolality was 245 mosm/kg with urine osmolality 519 mosm/kg. Chest X-ray normal. Abdominal X-ray showed faecal loading. Thyroid function tests were sent and results were consistent with severe hypothyroidism. TSH 141 mu/l (0.24.0), FT4 <4.0 and T3<0.7. Her ADH level was 4.89 pg/ml (appropriate being <1.5 pg/ml) which was osmotically inappropriate. Serum cortisol was 811 and Anti-TPO antibody titre >1000. The endocrinologist reviewed her and she was commenced on both levothyroxine and liothyronine. Aggressive fluid restriction was continued.
After 10 days, liothyronine was stopped and her levothyroxine was increased to 75 mcg a day. Her symptoms improved after starting treatment and her serum sodium level gradually increased to 134 mmol/l in two weeks when thyroid function tests were TSH 4.25 (0.24.0) and FT4 10.3 (12.026.6). She was discharged on levothyroxine 150 mcg a day. At 8-week clinic review, she was asymptomatic and her thyroid function tests were normal.
Comment: Physicians should be aware of the importance of checking thyroid function tests, when there is unexplained reduction in serum sodium concentration.
ADH measurement confirmed the diagnostic suspicion of SIADH with the patient responding very well to thyroid replacement and fluid restriction.