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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 synthroid to kick in? The doctors say it takes 3 weeks to start to notice a difference. They won't check your thyroid blood work until you've been on a dose for a minimum of 6 weeks. This is how long it takes to get into the part of your blood they use for the tests. When I started medication I noticed a big difference the first day, and was improved for the next 2 weeks.