Synthroid dosage: how to get your levothyroxine right
Many people take Synthroid and still feel off — often it's not the drug, it's the dose, timing, or interactions. This short guide explains how dosing works, what to watch for, and practical steps you can use with your doctor to reach the right dose faster.
How dosing works
Synthroid (levothyroxine) replaces the thyroid hormone your body lacks. The goal is a normal TSH on a blood test. For most adults, full replacement is about 1.6 mcg per kg of body weight per day. That usually equals roughly 75–125 mcg daily for a typical adult. Younger healthy adults often start near the full replacement dose.
If you're older or have heart disease, you start lower — commonly 25–50 mcg daily — and increase slowly to avoid straining the heart. Pregnant people usually need about a 25–30% higher dose once pregnancy is confirmed; doctors check TSH every 4 weeks early in pregnancy. Severe hypothyroidism or emergency cases use different plans — your clinician will handle those.
After any dose change, wait 6–8 weeks before repeating TSH because levothyroxine has a long half-life and blood levels take time to settle.
Practical tips and common mistakes
Take Synthroid on an empty stomach: 30–60 minutes before breakfast or at bedtime at least 3–4 hours after your last meal. Food, coffee, and some supplements cut absorption and can make the same pill act weaker.
Avoid calcium, iron, multivitamins with iron, antacids containing aluminum or magnesium, and some bile-acid binders within four hours of your dose. Soy, high-fiber diets, and certain prescription drugs can also interfere. If you switch brands or from brand-name to generic (or vice versa), get a TSH check in 6–8 weeks — small formulation differences can matter.
If you miss a dose, take it as soon as you remember unless it's close to your next dose. Don't double up to catch up. Signs you're underdosed include tiredness, weight gain, and cold intolerance. Overtreatment can cause palpitations, sweating, weight loss, anxiety, or bone thinning over time. If you notice chest pain, fast heartbeat, or sudden weight changes, call your doctor.
Special groups: babies and children need different dosing by weight and frequent checks. Elderly people are more sensitive and usually need lower doses. If you're pregnant, planning pregnancy, or breastfeeding, tell your doctor so they can adjust and monitor TSH more often.
Work with your doctor and your pharmacist. Bring a list of all pills and supplements, and ask for clear follow-up dates for TSH checks. Small steps — correct timing, watching interactions, and timely blood tests — often make the biggest difference in feeling better on Synthroid.
