There are many reasons.
Those conditions mentioned certainly cause tremors. Here's some more:
Hypoglycemia (low blood sugar)
Alcohol and other drug withdrawal
Chances are at your age it is blood sugar related. When it occurs, drink some fruit juice. If it lessens, you have your answer. If it doesn't see a doctor.
There are two tremor disorders that are generally benign (not harmful). One is familial tremors, other essential tremors. Familial, as it sounds is an inherited condition. It can be treated with medication, but can not be cured. The other condition, essential tremor is similar to familial but no family linkage can be found. It too can be treated but not cured.
More info below. Hope this helps.
Rick the Pharmacist
Essential tremor is a nerve disorder in which tremors (shakes) occur in a person who is moving or trying to move and no cause can be identified.
All people have some degree of tremor during movement. This shaking normally involves mainly the hands. Stress, fatigue, anger, fear, caffeine, and cigarettes may temporarily worsen this type of tremor.
Essential tremor is the most common form of abnormal tremor. Although the cause is unknown, new research shows that the part of the brain called the cerebellum does not appear to work correctly in patients with essential tremor. The cerebellum is the part of the brain that coordinates muscle movements.
Essential tremor is a relatively benign condition, affecting movement or voice quality, but with no other effects. It involves a rhythmic, moderately rapid tremor of voluntary muscles. Purposeful movements may make the tremors worse, while avoiding hand movements may make the tremors go away completely. People with essential tremors may have trouble holding or using small objects, such as silverware or a pen.
Over time, the tremors may affect the hands, arms, head, voice box, eyelids, or other muscles. An essential tremor rarely involves the legs or feet.
There are several different types of essential tremor, including young-onset essential tremor and essential tremor with head tremor. These types differ in their response to treatment.
If an essential tremor occurs in more than one member of a family, it is called a familial tremor. It appears that genes may play a role in the development of essential tremors. Not only have the tremors been shown to be passed down through families, but an identical twin (who shares the same genes) of a person with essential tremor is twice as likely as a fraternal twin (who has different genes) to have essential tremor. Since some identical twins do not share this condition, environmental factors must also play a role.
Essential tremors can occur at any age but are most common in people older than 65.