Levothyroxine

Nursing pharmacology study reference (NCLEX-style monograph). Numeric values are standard teaching ranges for study and must be verified against current manufacturer labeling before clinical use. This is educational content, not prescribing guidance.

Levothyroxine (brand names Synthroid, Levoxyl) — Thyroid hormone replacement; Synthetic T4.

Identification

• Therapeutic class: Thyroid hormone replacement.
• Pharmacologic class: Synthetic T4.

Pharmacology

• Mechanism of action: Replaces thyroxine to restore normal metabolic rate.
• Onset / peak / duration: Onset 3 to 5 days; full effect 4 to 6 weeks; long duration.
• Half-life / therapeutic level: Half-life about 7 days; goal is normal TSH.

Clinical use

• Indications: Hypothyroidism, myxedema, thyroid suppression.
• Usual dose, route, frequency: 25 to 200 mcg PO once daily on an empty stomach; IV at about half the oral dose.
• Maximum dose / adjustments: Individualized by TSH; start low in elderly and cardiac patients.

Safety

• Contraindications: Acute MI, untreated adrenal insufficiency, thyrotoxicosis.
• Black box warning: Not for weight loss; large doses can cause serious or life-threatening toxicity.
• Interactions: Calcium, iron, antacids, and bile acid sequestrants reduce absorption (separate by 4 hours); warfarin (increased effect); soy and high-fiber foods reduce absorption; espresso/coffee timing.
• Pregnancy / lactation: Continue and adjust; requirements often increase.
• High-alert: No.

Adverse effects

• Common side effects: Usually none at correct dose.
• Serious effects to report: Overtreatment signs (palpitations, tremor, weight loss, heat intolerance, insomnia), chest pain.
• Antidote / reversal: Dose reduction; beta blocker for acute overtreatment symptoms.

Nursing process

• Assessment before administration: TSH and free T4, heart rate, cardiac history.
• Interventions during therapy: Give consistently on an empty stomach; maintain the same brand.
• Monitor: TSH every 6 to 8 weeks after changes, heart rate, symptoms.
• Evaluation / expected outcome: Normal TSH and resolution of hypothyroid symptoms.

Patient teaching

• Patient teaching: Same time each morning, 30 to 60 minutes before food; do not switch brands without retesting.
• Notify provider if: Palpitations, chest pain, tremor, marked weight change.
• Administration tips: Empty stomach, morning; separate from calcium and iron.