- See oral and intravenous dosing below
-
Generic Levothyroxine is of similar quality to brand drugs
- However agents from different manufacturers are not bioequivalent
- When manufacturer changes, recheck TSH level in 6 to 8 weeks after change
- Levothyroxine alone is the only recommended replacement strategy
- T4 is converted in peripheral tissues to T3
- T3-T4 combination is not recommended
- Desiccated Thyroid Hormone is not recommended
- Grozinsky-Glasberg (2006) J Clin Endocrinol Metab 91(7): 2592-9 [PubMed]
- Available Formulations
- Capsules are also available under the trade name Tirosint
- Levothyroxine Tablets: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 and 300 mcg
- Tablets may be crushed for use in infants and children
- Oral
- Use lower dosing in Subclinical Hypothyroidism
- Dosing precautions
- Take Levothyroxine at a consistent time each day
- However, any time of day is just as efficacious as another
- Doses should be taken 1 hour before a meal or 4 hours after the last meal (e.g. at bedtime) to maximize absorption
- Commonly taken before breakfast or at bedtime
- However, consistently taking Levothyroxine in the same way every day is more important than an empty Stomach
- If patient can only take at mealtime, then consistently take with meals and adjust dosing
- Avoid within 4 hours of products that decrease Levothyroxine absorption (e.g. Calcium, iron, Multivitamins)
- See Drug Interactions above
- Take Levothyroxine at a consistent time each day
- Anticipated total dose (50 to 200 mcg/day in adults, maximum 300 mg/day)
- Child 0 to 6 months: 8 to 10 mcg/kg/day orally daily
- Child 6 to 12 months: 6 to 8 mcg/kg/day orally daily
- Child 1 to 5 years: 5 to 6 mcg/kg/day orally daily
- Child 6 to 12 years: 4 to 5 mcg/kg/day orally daily
- Child >12 years: 2 to 3 mcg/kg/day orally daily
- Adults: 1.6 mcg/kg/day (1.5 to 1.8 mcg/kg/day, or roughly 1 mcg/lb)
- Elderly: 1 mcg/kg/day
- Younger persons (no comorbid conditions)
- Usual starting dose: 75 to 100 mcg daily
- Options for initiating dosing
- Option 1
- Start at 75 to 100 mcg daily
- Option 2
- Start at 0.8 mcg/kg/day (50% of anticipated dose)
- Increase to 1.6 mcg/kg/day at 2 weeks
- Option 1
- Titrate dose based on Thyroid Stimulating Hormone
- Adjust dose by 12.5 to 25 mcg increments every 6 weeks until full replacement dose reached
- Typical dose
- Range: 100 to 200 mcg/day
- Maximum: 300 mcg/day
- Age over 50 years or history of heart disease
- Start at 25-50 mcg daily
- Adjust dose by 12.5 to 25 mcg increments every 6 weeks until full replacement dose reached
- Follow Thyroid Stimulating Hormone (TSH) closely
- Pregnancy
- See Hypothyroidism in Pregnancy
- Maintaining euthyroid state in pregnancy is critical
- Intravenous dosing
- Maintenance (patient unable to take oral dose for >7 days)
- Parenteral dose is 70-80% of usual oral dose
-
Myxedema Coma (Hypothyroid Coma)
- Load 300 to 500 mcg IV once
- Next: 50 to 100 mcg IV daily until patient able to take oral dosing
- Excessive Thyroid Replacement
- Osteoporosis
- Atrial Fibrillation
- Cardiac hypertrophy
- Increased Intraventricular septum thickness
- Increased Left Ventricular posterior wall thickness
- Increased End Diastolic Dimension
- Increased Left Ventricular Mass Index
- Decreased Exercise Tolerance
- Decreased VO2 Max
- Decreased Anaerobic threshold
- Considered safe in Lactation
- Considered safe in pregnancy
- See Hypothyroidism in Pregnancy
- Maintaining euthyroid state in pregnancy is critical
- Protocols for monitoring
- Monitoring every 6 to 8 weeks
- TSH not yet stabilized after initiation
- Recent change in Thyroid Replacement dosing
- Recent change in manufacturer
- Consider monitoring less frequently than annually (usually done annually in this group)
- Requires dose stable
- Age under 50 years with weight stable
- No comorbid condition
- Levothyroxine dose 125 mcg or less
- Pecina (2014) Am J Med 127(3): 240-5 [PubMed]
- Monitoring at least annually (when on stable dose)
- Age over 50 years
- Weight change
- Monitoring every 6 to 8 weeks
- Specific Testing
- Thyroid Stimulating Hormone
- Lags Levothyroxine dose change by 6 weeks
- Target adjusting TSH to the normal mid-range (<3 mg/dl)
- Thyroxine (T4)
- Lags Levothyroxine dose change by 1-2 weeks
- Thyroid Stimulating Hormone
- Foods that interfere with Levothyroxine absorption (lower levels)
- Regular use of these foods may require increased dose
- Grapefruit should be avoided regardless of time taken
- Walnuts
- Dietary Fiber
- Soy products including soybean flour
- Interfere with GI absorption (lower levels)
- Increase metabolism of Thyroxine (lowers levels)
- Medications interfere with T4 production and T3 conversion (lower levels)
- Lithium
- Amiodarone
- Medications containing Iodine
- Beta Adrenergic Agonists
- Glucocorticoids
- Medications increasing Protein binding (lowers levels)
- Pregnancy (high Estrogenic state)
- Oral Contraceptive
- Estrogen Replacement
- Medications decreasing Protein binding (raises levels)
- Furosemide (Lasix)
- Mefenamic Acid (Ponstel)
- Salicylates
- Androgens
- Decreased Serum Proteins with aging
- Nephrotic Syndrome
- Cirrhosis
- Protein-losing Enteropathy
- Levothyroxine (DailyMed)
- (2023) Presc Lett 30(7): 39-40
- Svec (2001) CMEA Medicine Lecture, San Diego
- Dong (1997) JAMA, 277:1205-13 [PubMed]
- Hueston (2001) Am Fam Physician 64:1717-24 [PubMed]
- Jonklaas (2014) Thyroid 24(12):1670-751 +PMID:25266247 [PubMed]
- Mercuro (2000) J Clin Endocrinol Metab 85:159 [PubMed]
- Singer (1995) JAMA 273:808-12 [PubMed]
- Surks (1995) N Engl J Med 333:1688-94 [PubMed]