APLN and Hyponatremia: However, it can be hypothesized that a hypothyroidism-related decrease in apelin levels may contribute to the development of hyponatremia in some susceptible hypothyroid patients; namely, if simultaneous ADH/AVP oversecretion also evolves, for example as a side effect of a medication, due to high external temperature or increased fluid intake, hyponatremia may become manifested.