Thyroid Function Test as First-line Investigation of Hyponatremia: Is It Necessary?

Abstract

Non-severe hypothyroidism has no strong association with hyponatremia. However, thyroid function tests (TFT) continue to be ordered as a first-line investigation. The objective of this study was to establish the prevalence of inpatient hyponatremia and to reiterate the association between thyroid disorders and hyponatremia based on study findings and current literature. A cross-sectional study on 3,478 in-patients adults of both gender admitted to a Malaysian tertiary hospital was carried out.  Age, gender, sodium levels, thyroid stimulating hormone (TSH), free thyroxine (fT4) and clinical notes between 1/11/17-31/12/17 were extracted from the hospital’s information system. Bivariate analysis was performed using Mann-Whitney U, Chi-square and Spearman correlation tests. 21.9% of inpatients had hyponatremia of which 52% were mild. 359 (69%) of hyponatremic patients had a TFT sent where 71.9% were euthyroid. Sodium level is significantly correlated with age (R=-0.257, P<0.001). No correlations were found between sodium and TSH and fT4. Sodium was significantly lower in the non-thyroidal illness (NTI) group compared to the hypothyroid group. No significant differences in sodium were found between the hypothyroid group and other thyroid states. Hence it was conclude that in hyponatremia, TFT is indicated when there is clinical evidence of severe hypothyroidism, suspicion of hypothalamus-pituitary-axis disorder or when no cause of SIADH is apparent. Otherwise, TFT is unnecessary and costly as a first-line investigation.

Keywords: hyponatremia, hypothyroidism, hypothyroid, non-thyroidal illness, sodium