Despite the fact that serum levels of magnesium are not the best indicator of adequate magnesium levels, some studies have shown that when magnesium deficiency was induced in humans, the earliest sign was decreased serum magnesium levels (hypomagnesemia). Over time serum calcium levels also began to decrease (hypocalcemia) despite adequate dietary calcium and increased parathyroid hormone (PTH) secretion. Usually increased PTH secretion quickly results in the mobilization of calcium from bone and normalization of blood calcium levels. As the magnesium depletion progressed, PTH secretion diminished to low levels. Along with hypomagnesemia, signs of severe magnesium deficiency included hypocalcemia, low serum potassium levels (hypokalemia), retention of sodium, low circulating levels of PTH, neurological and muscular symptoms (tremor, muscle spasms, tetany), loss of appetite, nausea, vomiting, and personality changes.  Hypercalcemia can cause magnesium deficiency and wasting.