Magnesium deficiency?

Question
Late winter/early spring of this year I noticed my 5 yr old AQHA gelding started becoming very jumpy, nervous, and was constantly spooking at things that had always been there. He had always been the complete opposite; very laid back and mellow.  I chalked it up to spring fever but as of June he hasn't come out of it. I've started doing some research, and found that magnesium deficiency can cause this type of behavior. Also, I've read magnesium def can be a result of insulin resistance which my gelding has.  When he was 3 he was having all sorts of problems, and to make a long story short I put him on a low carb/low starch diet and his symptoms disappeared.  BTW, he was tested for PSSM and that was negative. I went back thru some old labwork he had done around that time and saw that he was low in magnesium, sodium, and calcium; however the vets never mentioned that to me.
  I have started him on the magnesium supplement Quiessence. He's only been on it 2 weeks so I haven't really noticed a difference yet.  My question to you is, could his behavior be caused by anything else?  Also, what are the symptoms of calcium and sodium defs?  A few other bizzare symptoms he has is profuse sweating, dry psoriasis-like skin in his front armpits, and constant itching.  Any help would be greatly appreciated!

Answer
His behavior would either be caused by a dietary problem, or a behavior one. A behavior problem would be something that happened to him that hurt his confidence. It could be something as easy as a new pasturemate that dethroned him in the hierarchy, or an incident while riding, or grazing or really anytime. Maybe a new neighbor moved in and is throwing rocks at him while you are away. Or a new dog is barking at him constantly and making him nervous. Some big scary thing to him that made him rethink his security. If you can rule this out, then you can be pretty sure that it is a dietary condition. I would make sure he has a salt block at all times and I would rather it be a natural one like a redmond salt block than a man made pressed one. Was he worked hard prior to getting his labwork done? How much work does he get in an average week? Do you supplement electrolytes?
The dry skin in his arm pit area could be irritation from lying down. His hooves would rest there and can cause the calloused skin. If he lays on hard ground it could cause what you describe.
Heavy sweating can cause electrolyte problems. A horse sweats even more when it is not conditioned, in hot or humid weather, or when it is nervous. I would keep him supplemented with electrolytes, give him a good salt block and see if he gets better.
How is his weight? What kind of low starch feed is he on? Is he eating the minimum recommended daily amount if it? I mean, is he eating enough of the feed according to the manufacturer to be sure he is eating a balanced diet? If it says he needs six pounds according to his weight, is he really getting six pounds?

A sweaty horse is an itchy horse. My endurance horses are so itchy after a ride from all the sweat. Just try to keep him clean and get the sweat off of him. That should make him feel better.