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Thailand's Queen Sirikit (L) sits in a vehicle as she leaves Siriraj Hospital with King Bhumibol Adulyadej (not in picture) in Bangkok, Thailand, May 10, 2015. REUTERS/Kerek Wongsa

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BANGKOK (Reuters) - Thailand's 83-year-old Queen Sirikit has undergone medical tests and has "insufficient blood in the brain", the Royal Household Bureau said in a rare statement on her health on Friday.

Her husband, King Bhumibol Adulyadej, 88, is in failing health and is also being treated for various ailments at a Bangkok hospital.

The Thai royal succession has prompted worries about instability in a country that has witnessed 19 coups or attempted coups and at least 19 constitutions since a constitutional monarchy replaced an absolute one in 1932.

"An x-ray showed that there is still a trace of a former illness of insufficient blood in the brain as per in 2012 but the check found no new abnormalities," the palace said.

In a second statement late in the day, the palace said Queen Sirikit's medical tests were completed and yielded "satisfactory results".

Queen Sirikit, who has seldom been seen in public in recent years, went for a health check on Wednesday, it said.

The palace last released a statement on Queen Sirikit's health in 2014.

(Reporting by Panarat Thepgumapant and Patpicha Tanakasempipat; Writing by Amy Sawitta Lefevre)

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