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Thyroid Hormone ReplacementAka: Thyroid Replacement, Levothyroxine, Synthroid, Thyroxine Replacement, T4 Replacement

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  1. Indications
    1. Hypothyroidism
  2. Preparations
    1. Generic levothyroxine bioequivalent to brand drugs
    2. See oral and intravenous dosing below
  3. Medication Interactions (Take 4 hours apart)
    1. Interfere with GI absorption (lower levels)
      1. Cholestyramine
      2. Colestipol
      3. Ferrous Sulfate
      4. Sucralfate
      5. Aluminum hydroxide antacids
      6. Calcium Supplementation
    2. Increase metabolism of Thyroxine (lowers levels)
      1. Phenytoin
      2. Carbamazepine (Tegretol)
      3. Rifampin
      4. Phenobarbital
      5. Warfarin (Coumadin)
      6. Oral Hypoglycemic agents
    3. Medications interfere with T4 production (lower levels)
      1. Lithium
      2. Amiodarone
      3. Medications containing iodine
    4. Medications increasing protein binding (lowers levels)
      1. Pregnancy (high Estrogenic state)
      2. Oral Contraceptive
      3. Estrogen Replacement
    5. Medications decreasing protein binding (raises levels)
      1. Furosemide (Lasix)
      2. Mefenamic acid (Ponstel)
      3. Salicylates
      4. Androgens
      5. Decreased Serum Proteins with aging
      6. Nephrotic Syndrome
      7. Cirrhosis
      8. Protein-losing enteropathy
  4. Dosing (lower dosing in Subclinical Hypothyroidism)
    1. Anticipated total dose (50 to 200 ug/day)
      1. Children: up to 4 ug/kg/day (full replacement)
      2. Adults: 1.7 ug/kg/day
      3. Elderly: 1 ug/kg/day
    2. Younger persons (no comorbid conditions)
      1. Usual starting dose: 75 to 100 ug qd
      2. Options for initiating dosing
        1. Option 1
          1. Start at 75 to 100 ug qd
        2. Option 2
          1. Start at 0.8 ug/kg/day (50% of anticipated dose)
          2. Increase to 1.7 ug/kg/day at 2 weeks
    3. Age over 50 years or history of heart disease
      1. Start at 12.5 to 25 ug qd
      2. Increase by 25 to 50 ug increments every 4-6 weeks
      3. Follow Thyroid Stimulating Hormone (TSH) closely
  5. Special Circumstances
    1. Intravenous dosing
      1. Indicated if unable to take oral dose for >7 days
      2. Parenteral dose is 70-80% of usual oral dose
    2. Adding T3 to T4 may improve neuropsychiatric symptoms
      1. See Liothyronine (Cytomel)
      2. Recent study suggests no benefit
        1. Clyde (2003) JAMA 290:2952
  6. Adverse Effects: Excessive Thyroid Replacement
    1. Cardiac hypertrophy
      1. Increased Intraventricular septum thickness
      2. Increased Left Ventricular posterior wall thickness
      3. Increased End Diastolic Dimension
      4. Increased Left Ventricular Mass Index
    2. Decreased Exercise Tolerance
      1. Decreased VO2 Max
      2. Decreased Anaerobic threshold
  7. Monitoring
    1. Protocols for monitoring
      1. Monitoring every 6 to 8 weeks
        1. TSH not yet stabilized after initiation
        2. Recent change in Thyroid replacement dosing
      2. Monitoring annually
        1. Age over 50 years
        2. Weight change
      3. Monitoring less frequently than annually
        1. Age under 50 years with weight stable
        2. No comorbid condition
    2. Specific Testing
      1. Thyroid Stimulating Hormone
        1. Lags levothyroxine dose change by 6 weeks
        2. Target adjusting TSH to the normal mid-range
      2. Thyroxine (T4)
        1. Lags levothyroxine dose change by 1-2 weeks
  8. References
    1. Svec (2001) CMEA Medicine Lecture, San Diego
    2. Dong (1997) JAMA 277:1205
    3. Hueston (2001) Am Fam Physician 64:1717
    4. Mercuro (2000) J Clin Endocrinol Metab 85:159
    5. Singer (1995) JAMA 273:808
    6. Surks (1995) N Engl J Med 333:1688

Synthroid (C0728762)

ConceptsAmino Acid, Peptide, or Protein (T116) , Pharmacologic Substance (T121)
EnglishAbbot Brand of Levothyroxine Sodium, Synthroid
CreditsDerived from the NIH UMLS (Unified Medical Language System)



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