Tremors are repetitive patterns of involuntary muscle contraction and relaxation originating from disorder in the basal ganglia of the brain. See image of hand tremor (to left) courtesy of Antonio Piedade.
Rythmic shaking may affect various parts of the body including hands, arms, legs, trunk, tongue, or vocal cords.1 
Causes include malnutrition, drug adverse effects especially antipsychotics, environmental toxins, and conditions such as Parkinson disease and thyroid disease, which causes toxic levels of thyroid hormone.
What Are Tremors In Celiac Disease and/or Gluten Sensitivity?
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http://www.ninds.nih.gov/disorders/tremor/detail_tremor.htm ↩