Short answer: The Synthroid label gives a full adult replacement dose of 1.6 mcg/kg/day, taken once daily on an empty stomach, one-half to one hour before breakfast. Some adults, older people and people with heart disease start lower. The dose is raised in 12.5 to 25 mcg steps every 4 to 6 weeks until thyroid levels are normal. Children's starting doses depend on age and weight.
This is what the FDA label says. Follow your prescriber. Your prescriber may use a different dose or pace than the label's standard schedule.
What Synthroid is approved for: Synthroid is approved for adults and children, including newborns, as replacement therapy for hypothyroidism from any cause (thyroid, pituitary or hypothalamic, present at birth or acquired), and as an add-on to surgery and radioiodine to suppress TSH in TSH-dependent well-differentiated thyroid cancer. It is not approved to suppress benign thyroid nodules or nontoxic goiter in people with enough iodine, or for hypothyroidism during recovery from subacute thyroiditis (section 1).
Source
SYNTHROID (levothyroxine sodium) tablets, for oral use, prescribing information. AbbVie. DailyMed, version published February 29, 2024. DailyMed setid 1e11ad30-1041-4520-10b0-8f9d30d30fcc
Synthroid is taken as a single daily dose, on an empty stomach, one-half to one hour before breakfast (section 2.1).
It is taken at least 4 hours before or after drugs that interfere with its absorption, such as iron and calcium supplements and antacids. Taking it regularly within one hour of certain foods may call for a dose review (sections 2.1 and 17).
For children who cannot swallow tablets, the tablet is crushed, mixed into 5 to 10 mL of water and given right away by spoon or dropper. The mixture is not stored, and it is not given in foods that lower absorption, such as soybean-based infant formula (section 2.1).
The table below is the label's schedule, from the Dosage and Administration section. Numbers are copied exactly; the section is in brackets.
| Patient Population | Starting Dosage | Dosage Titration Based on Serum TSH or Free-T4 |
|---|---|---|
| Adults diagnosed with hypothyroidism | Full replacement dose is 1.6 mcg/kg/day. Some patients require a lower starting dose. | Titrate dosage by 12.5 to 25 mcg increments every 4 to 6 weeks, as needed until the patient is euthyroid. |
| Adults at risk for atrial fibrillation or with underlying cardiac disease | Lower starting dose (less than 1.6 mcg/kg/day) | Titrate dosage every 6 to 8 weeks, as needed until the patient is euthyroid. |
| Geriatric patients | Lower starting dose (less than 1.6 mcg/kg/day) |
| Age | Starting Daily Dosage Per Kg Body Weight* |
|---|---|
| 0-3 months | 10-15 mcg/kg/day |
| 3-6 months | 8-10 mcg/kg/day |
| 6-12 months | 6-8 mcg/kg/day |
| 1-5 years | 5-6 mcg/kg/day |
| 6-12 years | 4-5 mcg/kg/day |
| Greater than 12 years but growth and puberty incomplete | 2-3 mcg/kg/day |
| Growth and puberty complete | 1.6 mcg/kg/day |
Table 1 footnote: doses above 200 mcg/day are seldom required. A poor response to more than 300 mcg/day is rare and may point to missed doses, poor absorption, drug interactions or a mix of these.
People at risk of atrial fibrillation or with heart disease start lower and are titrated more slowly, to avoid worsening heart symptoms.
In children, the dose is adjusted every 2 weeks as needed based on TSH or free-T4. Infants up to 3 months at risk of heart failure start lower and step up every 4 to 6 weeks. Children at risk of hyperactivity start at one-fourth of the full replacement dose and add one-fourth each week until the full dose is reached.
Pregnancy (Table 3): with pre-existing hypothyroidism and TSH above the trimester range, the dose is raised by 12.5 to 25 mcg per day with TSH checked every 4 weeks, then returned to the pre-pregnancy dose right after delivery with TSH checked 4 to 8 weeks postpartum. New-onset hypothyroidism starts at 1.6 mcg/kg/day if TSH is 10 mIU per liter or higher, or 1.0 mcg/kg/day if below 10.
For primary hypothyroidism the dose is titrated until TSH is normal. For pituitary or hypothalamic hypothyroidism, TSH is not reliable, so free-T4 is used and kept in the upper half of the normal range.
The full effect of a given dose may not be reached for 4 to 6 weeks.
Adults: TSH is checked 6 to 8 weeks after any dose change, then every 6 to 12 months on a stable dose. Children: TSH and T4 are checked 2 and 4 weeks after starting, 2 weeks after any change, then every 3 to 12 months until growth is complete.
Biotin supplements can throw off thyroid blood tests, so the label says to stop them at least 2 days before TSH or T4 testing.
The Synthroid label gives no missed-dose instruction. Ask your prescriber or pharmacist what to do if a dose is missed.
In Dose Track you set up Synthroid as a protocol with the dose and schedule your prescriber gave you. A protocol can hold dose steps, each with its own dose and length in days or weeks, so a step-up plan like the one on the label can be entered once and the app moves to the next step on its own. Reminders follow the schedule, and every dose is logged with the date and time, so you can see which step you are on and when your last dose was. The app follows what you enter; it does not choose or change a dose.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.