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.
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.