Increased parathyroid activity results in …

hypercalaemia

Reduced parathyroid activity results in …

hypocalcaemia often with tetany

tetany (intemittent muscular spasm) because Ca has a huge roll of mantanence and enabling of nerve functioning and of skeletal muscle contraction and so if we have reduced activity or concentration of Ca you get an increase in excitability and action potentials which would not normally reach thresholds and now do