MAGOX 400 indicates that might be an oxide form of magnesium, which isn't absorbed/used well by the body... You might want to try a different form of magnesium that's absorbed better and see if that helps. Also, be sure to check potassium levels.
When will you have your thyroid scan? I had to go off my medication last fall for a scan and it was very difficult.
That said, muscle cramps can be a symptom of Graves Disease, so once you get back on your medication, hopefully, symptoms will be alleviated. If not, perhaps your dosage isn't correct.
Thyroid conditions can contribute to muscle cramps. Are you taking an anti-thyroid medication? If so, what medication and dosage and how long have you been taking it?
Magnesium and/or potassium deficiencies can also contribute.
Magnesium deficiency is one of the most prevalent causes of muscle cramps and studies show that many/most of us are deficient. You can ask your doctor to order magnesium testing - you need the RBC magnesium test - for the most accurate result. You can also talk to your doctor about taking a magnesium supplement. A supplement in the form of magnesium glycinate, citrate, or malate is best. The oxide form is less expensive, but it also tends to have more of a laxative effect and it's not as readily absorbed. I found a gel cap that works well for me; it contains all 3 forms.
If you've had recent blood work, a Complete Metabolic Panel will contain a result of potassium which will tell you whether your potassium level is adequate.