I have been taking Magnesium Malate pills even before taking iodine. The Magnesium Malate is nice but sometime you need quick absorption.
I experienced the same thing you did. Some time when after taking a lot of iodine, my heart starts beating fast, now if I take Magnesium Malate right away, it will ease the palpitations but only after 15 min. But if I apply Magnesium Chloride oil on my skin right away, the palpitations, and even the headache go away in seconds.
So now I wise up, I apply Magnesium oil as soon as I get up in the morning then wait for 30 minute before taking Iodine and whatever else.
I almost never get cramped while exercising but the other day I worked out after taking iodine. After the session, my legs felt like I just ran a marathon, the twitching and heaviness started to get worse. I knew right there that the muscles needed more Magneisum. So I applied the oil right away and the symptoms went away.
It seems taking Iodine requires even more Magnesium on top of everyone's already deficient condition.