Hi Eva-Maria,

this question is easier to answer if you give some information on the occurence and frequency of the tremor. Is it an action tremor? It could be enhanced physiological tremor or essential tremor. Is there a family history?

Because you asked: There are some metabolic causes of tremor, here are the most common ones:

Hyperthyroidism

Hyperparathyroidism

Hypocalcemia

Hyponatremia

Hypomagnesemia

Hypoglycemia

Further reasons could be pharmacotherapy (esp. antipsychotics) related, substance abuse/addiction and tumor. 

Best,

Julian

Here is a good resource with diagnostic algorithm.

http://www.aafp.org/afp/2011/0315/p697.html

X fragile syndrome

Most important cause in adolescent are drugs do enquire about the history and second cause to b ruled out is hyerthyroidism. Rest are as stated above