Perhaps you lot already know this, but I didn't know a low sodium level can go with an underactive thyroid. Anyway, this is a good example of very clear explaining (though I'm not familiar with the website).
My sodium is low and my GP told me that this can make blood glucose show false highs - the readings are high when the sugars actually aren't. I have type 1 diabetes and my BGs have been running high for ages. Now that I've searched, it seems that it's the other way round: high blood glucose can cause false low sodium readings. (So on the blood count, low sodium does go with high sugar, but she had it backwards. Or she was right and I have it backwards?)
I'm not dehydrated and haven't got water retention swelling, and kidney function is always fine on regular tests, so I looked at the possible causes of low sodium and was interested to see hypothyroidism. As far as I know I *have not* got anything wrong with thyroid function this year.
I was also interested to see chronic pain can cause it - but it says severe, and mine isn't that bad these days (I have fibromyalgia). At least I wonder what counts as severe. Now I want to know the mechanism by which pain lowers sodium!