Hyponatremia may be associated with Hypothyroidism (low thyroid levels/function), especially with severe hypothyroidism and myxedema. Myxedema is a constellation of symptoms associated with severe hypothyroidism including:
•Confusion
•Swelling
•Hypothermia (low body temperature)
•Bradycardia (slow heart rate)
•Hypotension (low blood pressure)
•Hypoglycemia (low blood sugar)
With this being said thyroid function is often checked, with a simple blood test, to be sure this is not the cause of the low sodium level.
In those with myxedema, it’s the decrease in cardiac output which leads to the release of ADH (anti diuretic hormone) via carotid sinus baroreceptors. The ADH release is the signal that tells the kidney to reclaim and hold on to water, which ultimately dilutes the sodium level in the blood. Slower kidney function (GFR) may also be contributing via the kidneys inability to excrete excess water properly.
It is interesting however, that often these hypothyroid cases mimic SIADH, with high, rather then low urine sodium, despite the low cardiac function.
In closing, we want to be sure that thyroid levels are tested as part of the workup for SIADH/Hyponatremia.
Useful links and sources:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4470237/#__sec2title
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4450783/
https://www.webmd.com/a-to-z-guides/myxedema-coma#2-3
https://www.aafp.org/afp/2000/1201/p2485.html
https://healthjade.net/myxedema-coma/