Men have both Androgens, male hormones, as well as Estrogens, female hormones. The male hormones determine male characteristics like facial hair, baldness, muscle development and so forth.

Normally in men, Estrogen takes a back seat and is present in lower amounts.

When there is a misbalance between the male and female hormones in men, they may develop feminizing characteristics. Notable among them is Gynecomastia – or development of enlarged breasts in males.

There are many causes of Gynecomastia including physiological, age-related, drugs and so forth. (1, 2, 3)

Physiological causes of Gynecomastia

Gynecomastia has physiological causes in new born boys and in pubertal adolescent boys.

In newborns, breast development may be accompanied by milk secretion. This happens due to the influence of maternal female hormones on the boy within the womb. This usually goes away in a week or two.

Adolescent boys develop breast buds that usually go away in a year or two. This is also due to the changes in hormone balances in them at puberty.

The female hormone in fact allows for bone elongation and growth. A rise of this hormone during puberty thus is accompanied by breast bud development in boys.

Age-related Gynecomastia

Elderly men often develop Gynecomastia. With age testotesone or male hormones decline in men and feminizing features of the female hormone like Gynecomastia thus appears in elderly men.

Drugs and Gynecomastia

Some drugs have been known to be linked to Gynecomastia. These include:-

• Antifungals (Ketoconazole)
• Heart drugs (Spironolactone)
• Antibiotics (metronidazole)
• Antacids (Cimetidine)
• Prostate cancer or enlargement drugs (flutamide, finasteride etc.)
• AIDS drugs (Efavirenz)
• Anti-anxiety drugs (diazepam)
• Tricyclic antidepressants, heart failure drugs (digoxin)

Other causes of Gynecomastia

1. Long-term liver disease - Liver disease over long periods of time may increase female hormone production that may lead to Gynecomastia.
2. Kidney failure and need for dialysis
3. Lack of testosterone or male hormones
4. Exposure to Estrogen of female hormones
5. Intake of anabolic steroids
6. Alcohol, Heroin, Methadone or Marijuana use
7. Radiation therapy and cancer chemotherapy especially for testicular and prostate cancer
8. Genetic disorders – Klinefelter’s syndrome causes malformation and underdevelopment of testes in boys and leads to Gynecomastia.
9. Thyroid disorders - especially an overactive thyroid.
10. Tumors - especially those that secrete female hormones estrogen. Tumors of testes, adrenal glands or pituitary gland can also lead to misbalance of male and female hormones.
11. Malnutrition or severe dieting – On malnutrition the testosterone levels drop but female hormone levels stay normal. This leads to Gynecomastia in some.
12. Some herbs like tea tree or lavender in shampoos and soaps have been associated with Gynecomastia due to their purported weak estrogen like action.
13. Unknown – Sometimes the cause of Gynecomastia may be unknown.

Breast cancer and Gynecomastia

Sometimes the actual reason behind breast enlargement may be an underlying tumor or cancer.

Breast cancer in men is rare but needs to be excluded when Gynecomastia is detected. Breast cancer shows up as a lump that is firm or hard with or without pain and discharge from the nipples.

Tumors and cancers also tend to show up as a single breast enlargement while most hormonal reasons for breast enlargement in males usually produce enlargement of both breasts.
 

Sources:

1. http://www.nlm.nih.gov/medlineplus/ency/article/003165.htm
2. http://www.mayoclinic.com/health/gynecomastia/DS00850/DSECTION=causes
3. www.merckmanuals.com/.../gynecomastia.html