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Research has shown that carnitine can potentially impair the action of thyroid hormone -- which can cause fatigue and weakness. While this would not be expected to affect people with normal thyroid function, if you have low or borderline-low thyroid levels, this may explain your symptoms. Use of acetyl-L-carnitine or other forms of carnitine is not advisable when thyroid levels are low -- a condition particularly common among women older than 60 years. This and several other potential side effects of acetyl-L-carnitine are described in the "Concerns and Cautions" section of's Acetyl-L-Carnitine Supplements Review >>

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