Levothyroxine (Synthroid)
Synthetic T4 thyroid hormone replacement. Very long half-life of ~7.5 days allows once-daily dosing. Must be taken on an empty stomach for consistent absorption.
Projected serum levels — 50 mcg (≈ 0.050 mg), once daily
Maintenance schedule: 50 mcg (≈ 0.050 mg) once daily (oral).
Loading schedule: 150 mcg on day 1, then 50 mcg (≈ 0.050 mg) once daily — reaching therapeutic levels sooner.
Modeled steady state after ~17 days: peak ≈ 0.38 mg, trough ≈ 0.35 mg body load. Population-based estimate over 54 days for a reference dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.
Key facts
- Category
- Thyroid Hormone
- Route modeled
- Oral
- Model confidence
- inferred
- Half-life
- 7 days
- Common dose
- 50 mcg (≈ 0.050 mg)
- Suggested maximum
- 200 mcg/day
- Reference dose range
- 0.025–200 mcg (single dose)
- Suggested cadence
- once daily
- Validated against
- 0.60 mg oral — Cmax 0.057 mg/L at 2 h
Documented interactions
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danger
Apigenin + Levothyroxine (Synthroid)
Apigenin may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free concentration changes…
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danger
Aripiprazole (Abilify) + Levothyroxine (Synthroid)
Aripiprazole (Abilify) may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free…
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danger
Armour Thyroid (Desiccated Thyroid Extract) + Levothyroxine (Synthroid)
Armour Thyroid (Desiccated Thyroid Extract) may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic…
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danger
Astaxanthin + Levothyroxine (Synthroid)
Astaxanthin may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free concentration changes…
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danger
Atomoxetine (Strattera) + Levothyroxine (Synthroid)
Atomoxetine (Strattera) may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free…
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danger
Atorvastatin (Lipitor) + Levothyroxine (Synthroid)
Atorvastatin (Lipitor) may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free…
Serum checks 159 modeled interaction pairings for levothyroxine (synthroid) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.
Research behind this entry
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Levothyroxine treatment of subclinical hypothyroidism, fatal and nonfatal cardiovascular events, and mortality
Cohort study of 3,093 patients found levothyroxine treatment of subclinical hypothyroidism reduced ischemic heart disease events and all-cause mortality, especially in those under 70.
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Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment
Large cohort study found levothyroxine treatment of subclinical hypothyroidism in pregnancy reduced pregnancy loss but increased risks of preterm delivery and gestational diabetes.
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2014 ATA guidelines for the treatment of hypothyroidism
Comprehensive clinical guideline recommending levothyroxine as first-line treatment for hypothyroidism, with detailed dosing and monitoring protocols.
3 published studies referenced in the app, each with a plain-language summary.