Sept 12 (Reuters) - A panel of outside advisers to the U.S. Food and Drug Administration on Friday voted 8-5 to recommend approval for Natpara, a hormone replacement therapy developed by NPS Pharmaceuticals Inc.

Shares of NPS, which were halted in regular trading, were down about 1 percent at $32.40 after hours. Since an FDA staff report to the panel was released last week, the shares have gained about 20 percent.

Natpara is designed to treat hypoparathyroidism, a condition in which the body's parathyroid gland does not secrete enough parathyroid hormone (PTH). The hormone works with vitamin D to regulate body calcium.

Low levels of PTH can cause tingling in the fingers and toes, muscle spasms, fatigue, muscle aches, hair loss, dry skin, headaches, mood swings and memory problems.

The condition can be caused by congenital disorders or surgery and is currently treated with high doses of calcium and vitamin D. Natpara is a bioengineered version of the hormone. About 180,000 people globally suffer from hypoparathyroidism, according to NPS. In about 40 percent of cases, the condition cannot be controlled with calcium and vitamin D.

That uncontrolled population is what NPS initially plans to target and it consists of about 20,000 patients in the United States.

(Reporting by Deena Beasley in Los Angeles; editing by Matthew Lewis)