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Take Synthroid with water only on an empty stomach. Take Synthroid 4 hours before or after. taking iron or calcium supplements, or antacids. Soy, cottonseed meal, walnuts, fiber, and. grapefuit juice can affect the absorption of Synthroid.
Re: how long does it take for levothyroxine to start working after taking it. I started on 25 mg of synthroid about 1 year ago (TSH level was about 9) and definitely felt positive effects, like more focused and energized, after about 3-4 months i was put on 50mg and still felt ok.