How much does levothyroxine build up with daily dosing?

At steady state on a daily schedule, the model's level is about 10.61 times what one dose gives, and it swings about 1.1:1 between the peak and the trough before the next dose. Longer gaps between doses mean less build-up and a bigger swing.

Is this chart my actual levothyroxine level?

No. It is a modeled estimate from a published half-life, using the same one-compartment model as the Dose Track app. Real levels vary with body weight, food, kidney and liver function and other factors, and only a lab test measures them. It is not medical advice.

Levothyroxine Half-Life: Peak and Steady State Chart

Q: When does levothyroxine peak after a dose?

Blood levels peak about 2.5 hours after a dose by mouth, according to Leggio GM et al. 2006. Individual timing varies with food, the amount and the person.

Q: How long does levothyroxine take to reach steady state?

About 35 days (~5 weeks) of regular dosing at the same amount and interval, which is 5 half-lives. With a half-life of about 7 days, levels reach about 50% of steady state after 7 days and about 90% after 23 days (~3.3 weeks). Changing the dose or interval starts a new climb toward a new steady state.

Q: How long does levothyroxine stay in your system after the last dose?

Roughly 5 half-lives, about 35 days (~5 weeks), until about 97% has cleared in the model. Half is gone about 7 days after levels start falling. Real clearance varies from person to person, and a drug can still have effects after levels are low.

Short answer: Levothyroxine has a half-life of about 7 days (Synthroid (levothyroxine sodium) label) and reaches steady state after about 35 days (~5 weeks) of regular dosing. Levels peak about 2.5 hours after a dose by mouth. On a daily schedule the model builds up to about 10.6× a single dose, with a peak-to-trough swing of about 1.1:1.

What do levothyroxine levels look like with daily dosing?

The Synthroid label gives a full adult replacement dose of about 1.6 mcg/kg/day. The chart uses 100 mcg once daily, one of the labeled tablet strengths, as an example; your dose depends on your labs and your prescriber. Source: Synthroid (levothyroxine sodium) label. Bioavailability 80% as in the app. Dashed line: 5 half-lives. Change the amount or schedule in the calculator.

When does levothyroxine peak after a dose?

Levels peak about 2.5 hours after a dose by mouth.

How long does levothyroxine take to reach steady state?

With regular dosing at a fixed amount and interval, levels climb toward steady state by the same fraction every half-life, whatever the dose. At a half-life of about 7 days:

Time from the first dose to each share of steady state, levothyroxine (half-life 7 days).
Share of steady state	Half-lives	Time
50%	1	7 days
75%	2	14 days
90%	3.3	23 days (~3.3 weeks)
97% (Dose Track's steady-state rule)	5	35 days (~5 weeks)

How much do levothyroxine levels swing between doses?

The longer the gap between doses compared with the half-life, the bigger the swing from peak to trough and the less the levels build up. These ratios do not depend on the amount.

Steady-state peak:trough and build-up by dosing interval, levothyroxine, in Dose Track's model.
Interval	Peak : trough	Build-up vs one dose
Daily	1.1 : 1	10.61×
Every other day	1.21 : 1	5.57×
Twice weekly	1.4 : 1	3.41×
Weekly	1.98 : 1	2×
Every 2 weeks	3.95 : 1	1.33×

Build-up is the accumulation ratio 1 / (1 − e^−k·τ), with k = ln 2 / half-life and τ the interval.

How long does levothyroxine stay in your system?

After the last dose, about half is gone every 7 days. After 5 half-lives, about 35 days (~5 weeks), roughly 97% has cleared in the model.

Where do these numbers come from?

The FDA Synthroid label gives a half-life for levothyroxine of 6 to 7 days in people with normal thyroid function, 3 to 4 days in hyperthyroidism and 9 to 10 days in hypothyroidism; Dose Track uses 7 days. The label gives no time to peak. In a study of healthy men given 100 mcg (Leggio 2006), levels peaked about 2.7 hours after a dose; the app uses 2.5 hours.

The curve uses the half-life, time to peak, route and bioavailability built into the Dose Track app for levothyroxine, and the same one-compartment model with first-order absorption. These are modeled estimates from published half-lives, drawn with the same one-compartment model as the Dose Track app. Real levels vary from person to person, and only a lab test measures them. This is not medical advice and not a dosing recommendation; dosing decisions belong with you and your prescriber.

Vial sizes and syringe math are in the Levothyroxine guide.

When does levothyroxine peak and reach steady state?