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levothyroxine sodium

Brand: Synthroid, Levoxyl, Tirosint, Unithroid, Euthyrox

⚠ BBW TDM Required Prototype Drug
Drug Class: thyroid hormone
Drug Family: thyroid agent
Subclass: synthetic T4 (thyroxine)
Organ Systems: endocrine

Mechanism of Action

Synthetic T4 is converted to T3 (triiodothyronine) in peripheral tissues by iodothyronine deiodinases; T3 binds nuclear thyroid hormone receptors, modulating transcription of hundreds of genes controlling metabolic rate, thermogenesis, cardiac output, and development.

thyroid hormone receptors (TR-alpha, TR-beta)

Indications

  • hypothyroidism (drug of choice)
  • myxedema coma (IV)
  • thyroid cancer (suppression therapy)
  • euthyroid goiter

Contraindications

  • thyrotoxicosis
  • uncorrected adrenal insufficiency (must treat concurrent AI before giving thyroid hormone to prevent adrenal crisis)
  • acute MI (relative; titrate very carefully)

Adverse Effects

Common

  • signs of hyperthyroidism with excessive dose: tachycardia, palpitations, tremor, weight loss, insomnia, heat intolerance

Serious

  • atrial fibrillation
  • cardiac ischemia (in elderly/CAD with excessive dose)
  • osteoporosis (suppression of TSH below normal — promotes bone resorption)
  • adrenal crisis (if started before treating concurrent adrenal insufficiency)

Pharmacokinetics (ADME)

Absorption Oral: 40–80% depending on formulation and administration; significantly reduced by food, calcium, iron, antacids — take on empty stomach, 30–60 min before food
Distribution Highly protein-bound (>99%); thyroxine-binding globulin (TBG) is primary carrier
Metabolism Peripheral deiodination to active T3 (by deiodinase type 1 and 2) and inactive reverse T3; hepatic glucuronidation and sulfation
Excretion Fecal (biliary) and renal
Half-life 7 days — allows once-daily dosing; levels stabilize after 5–6 weeks
Onset Several weeks
Peak 2–4 hours
Duration 1 week
Protein Binding >99%
Vd ~10 L

Drug Interactions

Drug / Agent Mechanism Severity
calcium, iron, antacids (aluminum/magnesium), sucralfate, proton pump inhibitors reduce levothyroxine oral absorption; separate by 4 hours major
cholestyramine / colestipol bind levothyroxine in gut, reducing absorption; separate by 4–6 hours major
warfarin thyroid hormones increase catabolism of vitamin K-dependent clotting factors; INR rises with hypothyroid-to-euthyroid transition major

Nursing Considerations

  1. Administer on an empty stomach 30–60 minutes before the first meal of the day; avoid calcium, iron, multivitamins, and antacids within 4 hours of levothyroxine
  2. Do not switch between brand names without monitoring — levothyroxine brands are not considered bioequivalent by the FDA; switching requires TSH recheck in 4–6 weeks
  3. Monitor TSH every 4–6 weeks after initiation or dose change until stable; target TSH range 0.5–2.5 mIU/L for most hypothyroid patients; lower targets (0.1–0.5) for thyroid cancer suppression
  4. In elderly patients or those with cardiac disease: initiate at 25–50 mcg/day and increase by 12.5–25 mcg every 4–6 weeks; aggressive initiation can precipitate angina or arrhythmias

Clinical Pearls

  • The 7-day half-life of levothyroxine means TSH will not fully reflect a new dose until 5–6 weeks after the change — checking TSH too early provides misleading results and leads to unnecessary dose adjustments
  • Adrenal insufficiency must be addressed before starting levothyroxine replacement in patients with secondary hypothyroidism — thyroid hormone accelerates cortisol metabolism and can precipitate addisonian crisis in patients with untreated AI

Safety Profile

Pregnancy use-with-caution
Lactation safe
Renal Adjustment Not required
Hepatic Adjustment Not required
TDM Required

Concordance Terms

Cross-referenced clinical concepts — click any term to see all content where it appears.

Euthyroid Goiter Hypothyroidism (drug Of Choice) Myxedema Coma (IV) Thyroid Cancer (suppression Therapy) Thyroid Hormone
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