Levothyroxine Dosage Calculator

For an otherwise healthy, non-elderly adult with primary hypothyroidism, the FDA-approved Synthroid label gives an average full replacement dose of approximately 1.6 mcg per kg per day. For example, a 70 kg adult is estimated around 112 mcg per day, which matches the label example of 100 to 125 mcg per day for a 70 kg adult. This is reference information only; the prescriber adjusts the actual dose based on serum TSH.

The Synthroid label recommends a conservative start. Elderly patients or patients with underlying cardiac disease start at 12.5 to 25 mcg per day, with increases every 6 to 8 weeks as needed until TSH normalizes; full replacement in the elderly may be below 1 mcg per kg per day. An older label variant gives 25 to 50 mcg per day as the initial dose for most patients over 50 or under 50 with cardiac disease. Never start at a full replacement dose when cardiac risk is present.

The American Thyroid Association 2017 guidelines recommend that women already on levothyroxine increase the dose as soon as possible after a missed menstruation or suspected pregnancy: either two additional tablets per week (nine instead of seven, about a 29 percent increase) or an increase of the daily dose by approximately 25 to 30 percent. Confirmatory thyroid testing is done at the same time, and the prescriber monitors TSH about every 4 weeks until weeks 16 to 20 of gestation. For TSH targets, the 2017 ATA guidelines recommend population- and trimester-specific reference ranges where available; where unavailable, an upper reference limit of approximately 4.0 mIU per L may be used. Note that fixed trimester ranges of 0.1 to 2.5, 0.2 to 3.0, and 0.3 to 3.0 were from the 2011 ATA guidelines and were retired in the 2017 update.

As a single daily dose, on an empty stomach, one-half to one hour before breakfast, per the Synthroid label section 2.1. Take it with water only, swallow the tablet whole, and keep the timing consistent every day. The peak effect of a given dose may not be seen for 4 to 6 weeks, so thyroid labs are not checked right after a dose change.

Iron and calcium supplements and antacids can lower levothyroxine absorption, so the tablet is taken 4 hours before or after these products. Soybean flour, cottonseed meal, walnuts, and dietary fiber may also reduce absorption, and grapefruit juice may reduce its effect. Proton pump inhibitors such as omeprazole can increase dose requirements, which is why the prescriber needs a complete medication and supplement list.

Coffee taken too soon can reduce levothyroxine absorption, so the commonly advised rule is to wait at least 30 to 60 minutes after the tablet before breakfast or coffee. Do not switch to taking the tablet with coffee or milk to save time; if morning timing is difficult, discuss a consistent bedtime schedule with the prescriber instead of changing it on your own.

If you remember on the same day, take it as soon as you remember, on an empty stomach and apart from food and interacting supplements. If it is already time for the next daily dose, skip the missed one and take only the scheduled dose; never double up to make up for a missed dose. If two or more doses were missed in a row, contact the prescribing clinician before deciding how to resume.

Common commercial tablet strengths are 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg. Because of this lineup, many target doses can be reached with a single tablet; the combination finder on this page searches single, two-tablet, and three-tablet combinations for the closest match to any target dose.

The label adjusts the dose in 12.5 to 25 mcg increments every 4 to 6 weeks until the patient is clinically euthyroid and serum TSH is normal. For elderly patients or those with cardiac disease, increases happen every 6 to 8 weeks; for severe longstanding hypothyroidism, increments of 12.5 to 25 mcg every 2 to 4 weeks. Doses above 200 mcg per day are seldom required; an inadequate response above 300 mcg per day is rare and usually points to poor compliance, malabsorption, or drug interactions.

No. Levothyroxine is a prescription medicine, and dosing is individualized by the prescriber based on periodic serum TSH assessment, age, weight, cardiac status, pregnancy, and interacting medicines. This tool restates label and guideline information for reference only and does not prescribe, adjust, or stop any treatment. Any dose decision belongs to the clinician in charge of care.