*This statement summarizes the third USPSTF recommendations for screening for chlamydial infection and the supporting scientific evidence, and it updates the 1995 recommendations contained in the Guide to Clinical Preventive Services, second edition (1).
Older adults may be more sensitive to the side effects of this drug, especially fast/pounding/irregular heartbeat. Current information shows that this drug may be used during pregnancy. Tell your doctor if you are pregnant because your dose may need to be adjusted. Levothyroxine passes into breast milk but is unlikely to harm a nursing infant.
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.