Because of the competing nature of calcium and magnesium, excessive calcium intake from foods or supplements can lead to a magnesium deficiency. The symptoms of magnesium deficiency are identical with many of the symptoms of thyroid disease, especially hyperthyroidism. People of Asian descent who get hyperthyroidism often become completely rigid and may be found lying in the streets this way. The condition is called hypokalemic periodic paralysis and is highly associated with hyperthyroidism. It's possible that the origin of the disease is not from low potassium (hypokalemia) but from low magnesium, which we know is a factor in hyperthyroidism.

Functions: 

The principal function of magnesium that is critical in thyroid disease is that it enables muscles to relax. With inadequate magnesium, the muscles cramp. When this happens to the heart muscles the heart does not go through a complete relaxation phase, and the next calcium-driven contraction begins before the relaxation is complete. This results in rapid heart beat and irregular heart rate known as arrhythmia.

Deficiency Effects and Symptoms:

According to the Nutrition Almanac, "Magnesium deficiency can easily occur because magnesium is refined out of many foods during processing. Cooking food removes the minerals; the oxalic acid in foods like spinach and phytic acid found in cereals bind magnesium in the body, as do unbalanced amounts of salts....A deficiency can occur in people with diabetes, those who use diuretics or digitalis preparations, the elderly, those with pancreatitis, chronic alcoholism, kwashiorkor, pregnancy, cirrhosis of the liver, arteriosclerosis or kidney malfunction, those on low-calorie or high-carbohydrate diets, and those who have severe malabsorption such as that caused  by chronic diarrhea or vomiting....Fluoride, high zinc levels, high levels of vitamin D, diuretics, and diarrhea will cause a deficiency of magnesium."

Also from N. A.: "Magnesium deficiency is thought to be closely related to coronary heart disease, including myocardial necrosis. An inadequate supply of this mineral may result in the formation of clots in the heart and brain and may contribute to calcium deposits in the kidneys blood vessels, and heart. Heart failure resulting from fibrillation and lesions in the small arteries is linked to a deficiency of magnesium, as is vasodilation, which is followed by hyperkinetic behavior and fatal convulsions."

"Symptoms of a deficiency may include gastrointestinal disorders, irregular heart rhythm, lack of coordination, muscle  twitch, tremors, weakness, apprehensiveness, personality changes, disorientation, confusion, depression, and irritability. A deficiency interferes with nerve and muscle impulses. Long-term deficiency can lead to tetany as in a calcium deficiency, alcoholic hallucinations, unusual face and eye movement, alopecia (baldness), swollen gums, and lesions of the gums."

Beneficial Effects:

N.A.: "Magnesium is vital in helping prevent heart attacks....After a heart attack, it has been found that supplementation provided a much higher survival rate and showed far less life-threatening dysrhythmias....It has also proved beneficial in the treatment of neuromuscular disorders, nervousness, tantrums, depression, sensitivity to noise, and hand tremor....Supplementing helps control dizziness, muscle weakness, twitching, heart disease, and high blood pressure,....reduce blood cholesterol and keep the arteries healthy....used for controlling convulsions in pregnant women, premature labor, and epileptic seizures."

Chocolate is a good source of magnesium and has a high magnesium to calcium ratio (about 4:1). Magnesium is low in hypers and the craving for chocolate may be driving my a need for magnesium. Unfortunately, chocolate may also be high in cadmium which is a probably negative factor for hyperT. Thus the craving for magnesium, if satisfied by eating chocolate, could exacerbate the symptoms of hyperT.

In this first study we see that magnesium is depleted in the  hyperthyroid state and treatment by Methimazole increases the magnesium content of both erythrocytes (red blood cells) and serum.  This emphasizes the need for magnesium supplementation in hyperthyroidism.

This second study examines the effect of magnesium on the sodium-potassium pump in the heart.  Magnesium deficiency is demonstrated to leave the number of pumps unaltered but to decrease the activity of the pumps.  This appears to result in an increase of sodium inside the cells with consequent arrhythmias in the heart.  This is evidence that supplementing magnesium can control rapid and irregular heartbeat experienced in hyperthyroidism.