By Toni Clarke

Jan 23 (Reuters) - The U.S. Food and Drug Administration has approved NPS Pharmaceuticals Inc.'s drug Natpara to treat a rare hormone disorder, validating a bet by Shire Plc , which recently agreed to acquire NPS for $5.2 billion.

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.

Analysts expect the drug to generate peak annual sales of $542 million by 2019, according to Thomson Reuters data.

Shire, which makes Adderall and Vyvanse for attention deficit hyperactivity disorder, is seeking to expand its position in the field of drugs for gastrointestinal disorders and rare diseases.

About 180,000 people globally suffer from hypoparathyroidism, according to New Jersey-based NPS. In about 40 percent of cases, the condition cannot be controlled with the current treatment of high doses of calcium and vitamin D.

That uncontrolled population is the audience that NPS initially plans to target, which in the United States is about 20,000 patients, the company said.

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.

Natpara is a bioengineered version of the hormone itself.

The condition can be caused by congenital disorders or surgery.

NPS also sells a drug for short-bowel syndrome, Gattex, which analysts on average expect to have peak sales of $526 million by 2019.

Data from a late-stage clinical trial of Natpara showed that 53 percent of patients treated with the drug were able to reduce their calcium and vitamin D supplements by 50 percent or more, compared with 2 percent of placebo-treated patients.

By the 24th week, 43 percent of patients treated with the drug were able to stop vitamin D therapy and required 500 mg a day or less of calcium, compared with only 5 percent of patients treated with placebo.

The drug was approved in Europe in 2006 to treat osteoporosis in post-menopausal women at high risk for fractures, under the trade name Preotact.

But European regulators noted that studies in rats indicated an increased incidence of osteosarcoma with long-term use and said that until further clinical data became available treatment should not be continued beyond 24 months.

The FDA declined to approve the drug for osteoporosis.

NPS withdrew Preotact from the market last year. NPS has filed for approval of Natpara in Europe.

(Reporting by Toni Clarke in Washington; Additional reporting by Deena Beasley in Los Angeles; Editing by Leslie Adler)