HONG KONG, April 30 (Reuters) - Hong Kong health officials warned residents on Monday against buying slimming products on the Internet after some were found to contain animal thyroid tissues and controlled substances.

Officials did not name the brands, but their contents came to light after four women were admitted to hospital over the past four months for heart palpitations, anxiety and trembling.

The four, aged between 14 and 29, had been using the products for between two weeks and six months. They have since recovered.

"People shouldn't depend on weight loss products and they definitely shouldn't buy such products from dubious sources," Henry Ng, principal medical and health officer with the Health Department, told a news conference.

A government laboratory found Sibutramine, a controlled drug, and animal thyroid tissues in all four slimming products.

Animal thyroid contains active hormones and excessive use may lead to confusion, coma, heart failure and death.

Sibutramine is used for weight reduction, but has been linked to side effects such as increasing heart rate and blood pressure, headaches, constipation, dizziness and insomnia.

"Because of the associated side effects, the use of anti-obesity medications should be supervised by medical practitioners," the government said in a statement.

As in many other places in the world, a svelte and slender figure is highly coveted in Hong Kong, where obesity is on the rise because of fast food and sedentary lifestyles.

Health experts however stress that the most effective way to shed extra pounds is to exercise and eat sensibly.

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