Effect of Optimal Thyroid Replacement Therapy on Chronic Hyponatremia with Focused Review of the Evidence, Mechanisms, and Clinical Implications

We present an unusual case of hyponatremia in an ambulatory hypothyroid patient and review related published literature on PubMed including, original articles, reviews and case reports that describe or refute the association and mechanism for the development of hyponatremia in hypothyroidism.

A 50-year-old female presented in ambulatory clinic with complaints of bilateral hand swelling, fatigue, dizziness, and unsteadiness while walking. Laboratory investigations revealed that she had hypothyroidism and hyponatremia. Thyroid hormone replacement therapy resulted in resolution of hypothyroidism symptoms as well as hyponatremia. A comprehensive search of related literature regarding the development of chronic hyponatremia in hypothyroidism revealed two schools of thought, which we have summarized in this report.

Based on our observations, we conclude that due to overlap in symptoms of hyponatremia and neurological manifestations of hypothyroidism, it is imperative to screen hypothyroid patients for underlying hyponatremia and treat accordingly in order to prevent long-term complications of chronic hyponatremia. Hyponatremia secondary to hypothyroidism resolves with appropriate thyroid hormone replacement therapy, which shows convincing evidence of an association between the two entities.

Central peptidergic neurons are hyperactive during collateral sprouting and inhibition of activity suppresses sprouting

Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS.