Eltroxin , Levoxyl, Levolet, Novothyrox, Synthroid, Unithroid
Classifications: hormone; thyroid hormone replacement;
Therapeutic: thyroid hormone replacement

Pregnancy Category: A


25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg tablets; 200 mcg, 500 mcg injection


Synthetically prepared levo-isomer of thyroxine (T4), with similar actions and uses (thyroxine, principal component of thyroid gland secretions, determines normal thyroid function). Principal effects include diuresis, loss of weight and puffiness, increased sense of well-being and activity tolerance, and rise of T3 and T4 serum levels toward normal.

Therapeutic Effect

By replacing decreased or absent thyroid hormone, it restores metabolic rate of a hypothyroid individual.


Specific replacement therapy for diminished or absent thyroid function resulting from primary or secondary atrophy of gland, surgery, excessive radiation or antithyroid drugs, congenital defect. Administered orally for hypothyroid state; administered IV for myxedematous coma or other thyroid dysfunctions demanding rapid replacement, as well as in failure to respond to oral therapy.


Hypersensitivity to levothyroxine; thyrotoxicosis; severe cardiovascular conditions, acute MI; obesity treatment; adrenal insufficiency.

Cautious Use

Cardiac disease, angina pectoris, cardiac arrhythmias, hypertension; diabetes mellitus; older adult, impaired kidney function, pregnancy (category A).

Route & Dosage

Thyroid Replacement
Adult: PO 25–50 mcg/d, gradually increased by 50–100 mcg q1–4wk to usual dose of 100–400 mcg/d IV/IM ? established oral dose (usually 50–100 mcg daily)
Child: PO 0–6 mo, 8–10 mcg/kg/d or 25–50 mcg/d; 6–12 mo, 6–8 mcg/kg/d or 50–75 mcg/d; 1–5 y, 5–6 mcg/kg/d or 75–100 mcg/d; 6–12 y, 4–5 mcg/kg/d or 100–150 mcg/d; >12 y, 2–3 mcg/kg/d or >150 mcg/d

Myxedematous Coma
Adult: IV 200–500 mcg day 1, additional 100–300 mcg on day 2 if needed


  • Give as a single dose, preferably 1 h before or 2 h after breakfast, to prevent insomnia. Give consistently with respect to meals.
  • Maintenance dosage for older adults may be 25% lower than for heavier and younger adults.
  • Store in tight, light-resistant container.

PREPARE: Direct: Reconstitute vial by adding 5 mL of NS for injection to each 100 mcg. Shake well to dissolve. Use immediately.  

ADMINISTER: Direct: Give bolus dose over 1 min.  

INCOMPATIBILITIES Do not mix with other medications.

  • Store dry powder at 15°–30° C (59°–86° F).

Adverse Effects (≥1%)

CNS: Irritability, nervousness, insomnia, headache (pseudotumor cerebri in children), tremors, craniosynostosis (excessive doses in children). CV: Palpitations, tachycardia, arrhythmias, angina pectoris, hypertension. GI: Nausea, diarrhea, change in appetite. Urogenital: Menstrual irregularities. Body as a Whole: Weight loss, heat intolerance, sweating, fever, leg cramps, temporary hair loss (children).


Drug: Cholestyramine, colestipol decrease absorption of levothyroxine; epinephrine, norepinephrine increase risk of cardiac insufficiency; oral anticoagulants may potentiate hypoprothrombinemia.


Absorption: Variable and incompletely absorbed from GI tract (50–80%). Peak: 3–4 wk. Duration: 1–3 wk. Distribution: Gradually released into tissue cells. Half-Life: 6–7 d.

Nursing Implications

Assessment & Drug Effects

  • Monitor pulse before each dose during dose adjustment. If rate is >100, consult physician.
  • Monitor for adverse effects during early adjustment. If metabolism increases too rapidly, especially in older adults and heart disease patients, symptoms of angina or cardiac failure may appear.
  • Note: Levothyroxine may aggravate severity of previously obscured symptoms of diabetes mellitus, Addison's disease, or diabetes insipidus. Therapy for these disorders may require adjustment.
  • Lab tests: Baseline and periodic tests of thyroid function. Closely monitor PT/INR and assess for evidence of bleeding if patient is receiving concurrent anticoagulant therapy. A decrease in anticoagulant dosage may be needed 1–4 wk after concurrent levothyroxine is started.
  • Monitor bone age, growth, and psychomotor function in children.
  • Some children have partial hair loss after a few months; it returns even with continued therapy.
  • Synthroid 100 and 300 mcg tablets contain tartrazine, which may cause an allergic-type reaction in certain patients; particularly those who are hypersensitive to aspirin.

Patient & Family Education

  • Thyroid replacement therapy is usually lifelong.
  • Learn how to self-monitor pulse rate. Notify physician if rate begins to increase above 100 or if rhythm changes are noted.
  • Notify physician immediately of signs of toxicity (e.g., chest pain, palpitations, nervousness).
  • Avoid OTC medications unless approved by physician.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

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