Synthroid (levothyroxine): Practical Guide

Synthroid is the brand name for levothyroxine, a synthetic thyroid hormone used to treat underactive thyroid (hypothyroidism). If your doctor prescribed it, this quick guide tells you what to do, what to watch for, and how to avoid common mistakes.

How it helps: Synthroid replaces missing thyroid hormone so your energy, mood, temperature, and metabolism return toward normal. Most people notice gradual improvement in fatigue, cold sensitivity, and constipation over weeks to months.

How to take Synthroid

Take Synthroid on an empty stomach, first thing in the morning, at least 30 to 60 minutes before breakfast. Swallow the tablet with a full glass of water. If you prefer evening dosing, take it at least three hours after your last meal. Consistency matters—use the same routine every day.

Avoid taking Synthroid within four hours of calcium supplements, iron pills, antacids with aluminum or magnesium, bile-acid binders, or sucralfate. Soy products and high-fiber meals can also lower absorption. If you need calcium or iron, take them at a different time of day.

Watch-outs & monitoring

Your doctor will check TSH (and sometimes free T4) about 6 to 8 weeks after starting or changing the dose. That timing lets your body reach a steady level. Once stable, checks are usually every 6 to 12 months unless symptoms change, you become pregnant, or your medications change.

Start doses vary. Younger healthy adults often start around 50 to 100 micrograms daily. Older adults or people with heart disease usually start much lower (12.5–25 mcg) and increase slowly. Never change the dose on your own.

Switching brands or to a generic can change how much drug your body gets. If your pharmacy changes the brand, ask your doctor if you should recheck your TSH after the switch. Some patients do better when they stick with the same manufacturer.

Side effects usually come from taking too much. Symptoms include fast heart rate, anxiety, sweating, weight loss, and trouble sleeping. If you feel these, call your doctor—do not stop the pill suddenly unless instructed. If you miss one dose, take it as soon as you remember that day and return to your schedule; don’t double up the next day.

Other important notes: Synthroid can interact with blood thinners like warfarin, diabetes meds, and certain antidepressants. Pregnancy often requires a higher dose; tell your doctor if you’re pregnant or planning to be. Keep a current list of all meds and supplements and bring it to appointments.

Bottom line: Take Synthroid consistently on an empty stomach, avoid certain supplements and foods near the dose, and get TSH checked 6–8 weeks after any change. Talk openly with your doctor about symptoms and any pharmacy brand changes.

Store Synthroid at room temperature away from moisture and heat. If you notice chest pain, severe shortness of breath, or fainting after a dose, seek emergency care. Keep pills in original bottle and check expiration dates. If you're unsure about interactions with a new prescription, call your pharmacist before starting, and save contact numbers for emergencies.