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Thyroid Hormone Replacement
Aka: Thyroid Hormone Replacement, Thyroid Replacement, Levothyroxine, Synthroid, Thyroxine Replacement, T4 Replacement- See Also
- Indications
- Preparations
- See oral and intravenous dosing below
- Generic Levothyroxine are of similar quality to brand drugs
- However agents from different manufacturers are not bioequivalent
- When manufacturer changes, recheck TSH level in 6 weeks after change
- Levothyroxine alone is only recommended replacement strategy
- T3-T4 combination is not recommended
- Desiccated Thyroid Hormone is not recommended
- Grozinsky-Glasberg (2006) J Clin Endocrinol Metab 91(7): 2592-9 [PubMed]
-
Drug Interactions (Take 4 hours apart)
- 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)
- Bile Acid Sequestrants (e.g. Cholestyramine, Colestipol)
- Ferrous Sulfate
- Orlistat
- Sucralfate
- Aluminum hydroxide or Magnesium HydroxideAntacids
- Calcium Supplementation (e.g. Calcium Carbonate)
- Proton Pump Inhibitors
- Cation-Exchange Resin (e.g. Sodium Polystyrene Sulfonate)
- 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
- Foods that interfere with Levothyroxine absorption (lower levels)
- Dosing
- 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)
- Commonly taken before breakfast or at bedtime
- Avoid within 4 hours of products that decrease Levothyroxine absorption
- See Drug Interactions above
- Take Levothyroxine at a consistent time each day
- Anticipated total dose (50 to 200 mcg/day)
- Children: up to 4 mcg/kg/day (full replacement)
- Adults: 1.6 mcg/kg/day (1.5 to 1.8 mcg/kg/day)
- 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
- Age over 50 years or history of heart disease
- Start at 25-50 mcg daily
- Increase by 25 mcg increments every 4 weeks until full replacement dose reached
- Follow Thyroid Stimulating Hormone (TSH) closely
- Pregnancy
- See Hypothyroidism in Pregnancy
- Maintaining euthyroid state in pregnancy is critical
- Management: Intravenous dosing
- Indicated if unable to take oral dose for >7 days
- Parenteral dose is 70-80% of usual oral dose
- Adverse Effects: 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
- Monitoring
- 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
- Protocols for monitoring
- References
- 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]