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Levothyroxine (Synthroid, Levoxyl, Tirosint)

Thyroid hormone replacement (synthetic T4)

Mechanism of action

Synthetic L-thyroxine (T4) that the body converts to active T3, restoring normal thyroid hormone signaling. Used for primary, secondary, and tertiary hypothyroidism, after thyroidectomy or radioactive iodine ablation, congenital hypothyroidism, and TSH suppression in some thyroid cancers.

Adverse effects

Life-threatening / NCLEX-tested

Iatrogenic thyrotoxicosis from over-replacement — atrial fibrillation, angina, tremor, weight loss, heat intolerance, anxiety, insomnia
Acceleration of pre-existing cardiovascular disease — start LOW and go SLOW in older adults and patients with coronary artery disease
Bone loss / osteoporosis with long-term over-suppression of TSH
Adrenal crisis if given to a patient with untreated adrenal insufficiency — replace cortisol FIRST

Side effects

Common — what to teach

Mild palpitations or tremor (often signal slight over-replacement)
Insomnia
Mild GI upset
Hair loss in the first 1–3 months (often improves)
Heat intolerance, sweating
Mild headache

Food & drug interactions

Calcium, iron, magnesium, antacids (aluminum/magnesium-containing), bile acid sequestrants (cholestyramine), sucralfate, soy products, and high-fiber meals all REDUCE absorption — separate by at least 4 hours. Many drugs RAISE replacement requirements: estrogens, phenytoin, carbamazepine, rifampin. Warfarin INR rises (more clotting factor turnover). Diabetic patients may need higher insulin/oral agent doses once euthyroid.

Nursing implications

Assessment, monitoring, patient teaching

TAKE ON AN EMPTY STOMACH, with water, FIRST THING IN THE MORNING, at least 30–60 minutes before food, coffee, or other meds — absorption is the central teaching point
Separate from calcium, iron, antacids, and PPIs by at least 4 hours
Monitor TSH 6–8 weeks after start or any dose change; once stable, every 6–12 months
Counsel against switching brand vs generic without rechecking TSH — small bioavailability differences matter at this narrow window
Older adults and patients with CAD: start at low doses (12.5–25 mcg) and titrate slowly to limit cardiac stress
Never abruptly stop — symptoms return slowly but reliably (half-life ~7 days)
If adrenal insufficiency is possible (panhypopituitarism, Addison's), replace cortisol BEFORE thyroid hormone — otherwise you can precipitate adrenal crisis
Pregnancy: requirements rise ~30% in first trimester — recheck TSH early

When to hold / contraindications

Suspected acute MI, unstable angina, or new tachyarrhythmia related to over-replacement
Confirmed thyrotoxicosis from over-replacement (TSH suppressed, free T4 elevated, symptomatic)
Untreated adrenal insufficiency — treat with cortisol first, then start levothyroxine
Severe acute illness with hemodynamic instability — discuss with the team
Known hypersensitivity (rare; usually to the inactive ingredients)

Memory anchor

Levothyroxine = "empty stomach, first thing, alone." Wait 30–60 min before coffee/food, and 4 hours from calcium/iron. Recheck TSH every 6–8 weeks after a dose change. Cortisol BEFORE thyroid hormone in suspected adrenal insufficiency.

Reminder: Drug cards are study aids, not clinical guidance. Always cross-check doses, holds, and contraindications with your facility's formulary and your clinical instructors before patient care.

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