Brief Answer:
Low testosterone

Detailed Answer:
Lupron given long term lowers the body's testosterone levels.

Testosterone is the most important sex hormone that men have. It is responsible for the typical male characteristics, such as facial, pubic, and body hair as well as muscle. This hormone also helps maintain sex drive, sperm production, and bone health. The brain and pituitary gland (a small gland at the base of the brain) control the production of testosterone by the testes.
In the short term, low testosterone (also called hypogonadism) can cause
A drop in sex drive
Poor erections
Low sperm count
Enlarged breasts
Over time, low testosterone may cause a man to lose body hair, muscle bulk, and strength and to gain body fat. Chronic (long-term) low testosterone may also cause weak bones (osteoporosis), mood changes, less energy, and smaller testes. Signs and symptoms (what you see and feel) vary from person to person.

May I ask if you have prostate cancer? And are you taking Lupron shots for treating this? This is also called Androgen Deprivation Therapy (ADT)
The male hormone testosterone and its related hormones, called androgens, can cause prostate cancer to grow. ADT stops the production of these hormones. ADT can be permanent by surgically removing the testicles. More often, though, it is temporary by using certain medications to keep the testes from making testosterone.
Doctors use ADT mainly to treat prostate cancer that has spread outside the prostate. ADT can cause a large loss of bone density, making the bones weak and more likely to break (fracture).
Because the risk of bone loss with ADT is great, if you are receiving or will receive treatment with ADT, ask your doctor if you should have a dual energy X-ray absorptiometry (DEXA) scan. This imaging test measures your bone mineral density (BMD), the amount of bone you have in a given area, such as your spine or hip. Men with low BMD at the start of ADT, or who lose bone during ADT, may benefit from medications designed to prevent bone loss and osteoporosis.

Why is bone loss dangerous?
In the United States, at least two million men have osteoporosis. About one in five men over age 50 will have a bone fracture related to osteoporosis that will seriously affect their quality of life. Osteoporosis in men can be a major health problem, above all among those who break a hip. Elderly men are much more likely than women to die or have chronic (long-term) disability after a hip fracture.

So you may no wish to know how can you prevent bone loss during and after prostate cancer treatment:
The most effective way to prevent bone loss during treatment for prostate cancer is by using medications called bisphosphonates. Bone loss is greatest early in the course of ADT treatment, so using bisphosphonates to prevent osteoporosis may be most effective if started in the first year. You may need to keep taking these medications for a few years to protect your bones.
Apart from cancer treatment, men with prostate cancer may have bone loss due to other causes. These may include lack of physical activity, tobacco use and drinking too much alcohol, a diet low in calcium and vitamin D (which is needed for calcium to be absorbed into bone), anti-seizure medications, and glucocorticoids (such as prednisone) used to treat other conditions.
If you have prostate cancer, you should consider the health of your bones when making decisions about cancer treatment. Discuss with your doctor your risk of bone loss and ways you can prevent osteoporosis.

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