Levothyroxine dosage
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levothyroxine dosing hypothyroidism
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Here is a comprehensive summary of levothyroxine dosing from authoritative sources:
Levothyroxine Dosage
Available Formulations
- Tablets: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg
- Capsules (Tirosint): 13–200 mcg range
- Oral solution: 100 mcg/5 mL; unit-dose ampules
- Injection: 100, 200, 500 mcg (lyophilized powder); IV dose = ~80% of oral dose
Hypothyroidism — Adult Dosing
| Scenario | Starting Dose | Target / Notes |
|---|---|---|
| Young/healthy adults, mild disease | Full replacement immediately | ~1.7 mcg/kg/day (avg ~125 mcg/day) |
| Adults >50 years, no cardiac disease | 50 mcg/day | Titrate up |
| Elderly / cardiac disease | 12.5–25 mcg/day × 2 weeks, then ↑ by 12.5–25 mcg/day q2 weeks | Heart is highly sensitive to T4 levels |
| Post-thyroidectomy suppression (thyroid cancer) | — | ~2.2 mcg/kg/day (goal: suppress TSH) |
| Pregnancy | Higher dose usually required | Due to elevated TBG, placental Dio3; revert to pre-pregnancy dose the day after delivery |
| Bariatric surgery / malabsorption | Higher dose may be needed | Atrophic gastritis, celiac disease, H. pylori also increase requirement |
| Older adults >65 years | ~1.6 mcg/kg/day | Body mass declines |
Usual maintenance range: 100–200 mcg/day (oral)
IV dosing: 50–75% of oral dose once daily (or 80% per Goodman & Gilman)
Goal: Normalize serum TSH (primary hypothyroidism) or free T4 (secondary/tertiary hypothyroidism). TSH target: typically 0.5–2.5 mIU/L.
Myxedema Coma (Adult)
- 300–500 mcg IV × 1 loading dose, then 50–100 mcg IV once daily
- Convert to oral once stabilized
Pediatric Dosing (Hypothyroidism, PO)
| Age | Dose |
|---|---|
| 1–3 months | 10–15 mcg/kg/day (up to 12–17 mcg/kg/day if T4 very low; start 25 mcg/day if cardiac risk) |
| 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 |
| >12 years (pre-puberty, incomplete growth) | 2–3 mcg/kg/day |
| >12 years (complete growth/puberty) | 1.7 mcg/kg/day |
Pediatric IM/IV: 50–75% of oral dose
Titration & Monitoring
- Steady-state is not reached until 6–8 weeks after a consistent dose — do not adjust sooner
- Recheck TSH and free T4 ~6 weeks after any dose change
- In children, also monitor growth and development
- Dose by lean body mass (not total weight)
Administration
- Take on an empty stomach (30–60 min before meals, 4 h after meals, or at bedtime) — reduces TSH variability
- Administer tablets with a full glass of water
- Separate by ≥4 hours from: calcium, iron, antacids (reduce absorption)
- Liquid capsules may improve absorption in GI disease
Drugs/Factors That Alter Dosage Requirements
| Increase requirement | Decrease requirement |
|---|---|
| Pregnancy, estrogen, TKIs, malabsorption, bariatric surgery, celiac, H. pylori | Advancing age (>65), androgen therapy in women |
| Phenytoin, rifampin, carbamazepine, orlistat | — |
| Calcium/iron supplements, antacids, grapefruit juice | — |
| Amiodarone, propranolol (inhibit T4→T3 conversion) | — |
Key Equivalence
- 100 mcg levothyroxine ≈ 65 mg desiccated thyroid USP
- Levothyroxine monotherapy is preferred over T4+T3 combination (multiple RCTs show no superiority of combination therapy for most patients)
Contraindications
- Acute MI, thyrotoxicosis, uncorrected adrenal insufficiency
Sources:
- Goodman & Gilman's Pharmacological Basis of Therapeutics, Ch. 47
- Katzung's Basic and Clinical Pharmacology, 16th ed., Ch. 38
- Harriet Lane Handbook, 23rd ed. (pediatric dosing)
What are the guidelines for missed dose
The previous context already contains the relevant information from Goodman & Gilman's. Here is the guideline:
Levothyroxine — Missed Dose
Because levothyroxine has a plasma half-life of approximately 7 days, missing a single dose has only a marginal effect on serum TSH and free T4 levels.
What to Do
- Take a double dose the next day to maintain consistent overall dosing.
- There is no need to take the missed dose as soon as you remember if it is already the next day — simply double up on the following dose.
Why This Works
The very long half-life means serum T4 levels decline slowly and the TSH barely changes after one missed dose. This also means:
- Once-daily dosing is sufficient (no need for multiple daily doses)
- Skipping a day occasionally is not clinically dangerous, but consistent adherence matters for maintaining stable TSH levels
"Omission of one day's dose has only marginal effects on the serum TSH and free T4, but to maintain consistent dosing, the patient should be instructed to take a double dose the next day." — Goodman & Gilman's Pharmacological Basis of Therapeutics, Ch. 47
Can the patient take missed dose as soon as he remembers
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levothyroxine missed dose take as soon as remembered guidelines
Yes — with a condition. The standard clinical guidance is:
Levothyroxine Missed Dose — "Take As Soon As Remembered" Rule
| Situation | What to Do |
|---|---|
| Remembered same day | Take the missed dose as soon as remembered, even if it means taking it later in the day (with food if necessary) |
| Remembered the next day | Take a double dose that day instead (do not go back and take the previous day's dose separately) |
| Almost time for the next scheduled dose | Skip the missed dose; take the next dose as usual — do not double up at that point |
Key Reasoning
Levothyroxine's 7-day half-life means a single missed dose causes only a marginal drop in serum T4 and TSH. This pharmacokinetic property makes same-day make-up dosing safe and the double-dose strategy on the following day an acceptable catch-up strategy.
"Omission of one day's dose has only marginal effects on the serum TSH and free T4, but to maintain consistent dosing, the patient should be instructed to take a double dose the next day." — Goodman & Gilman's Pharmacological Basis of Therapeutics
The textbook specifically describes the next-day double dose as the recommended approach, but this applies when the patient does not remember until the following day. If remembered the same day, taking the dose promptly (even with food) is both safe and preferred to skipping entirely.
Practical Reminder
- Absorption is slightly reduced when taken with food, but this is far better than missing the dose altogether given the long half-life
- Consistent daily dosing on an empty stomach remains the ideal for stable TSH levels over time
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