What causes are of temporary gynecomastia?

The first thing that: Comes to mind is medications.

Obesity,meds,hormone: Obesity or excess weight gain, hormonal imbalance, medication side effects can all result in gynecomastia.

Many possibilities: In this age group, the most common causes are marijauna use and anabolic steroid use in bodybuilders. Other less common causes include liver disease, certain medications, testicular tumors, and thyroid imbalances. Occassionally there is no explanation (idiopathic) and the problem resolves on its own.

Many Causes: Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. Although there may be many “causes”, most cases are idiopathic (no specific cause). The use of certain medications, drugs, organ dysfunction (kidney or liver), hormonal imbalances etc. May also be causative.