Levothyroxine

Overview

Levothyroxine is a synthetic form of thyroxine (T4), the principal hormone produced by the thyroid gland. In tissues, T4 is converted by deiodinase enzymes to triiodothyronine (T3), the biologically active hormone that binds nuclear thyroid receptors to regulate gene expression. Adequate thyroid hormone is essential for metabolic rate, thermogenesis, cardiac function, neurocognitive performance, mood, fertility, and normal growth and development in children. When thyroid output is deficient—most commonly due to autoimmune thyroiditis (Hashimoto’s), prior thyroid surgery, radioiodine, or congenital absence—levothyroxine replaces what the gland would normally produce.

Pharmacologically, levothyroxine has predictable oral absorption (optimised fasting), extensive protein binding, and a long half-life (~7 days), which supports once-daily dosing. Because the hormone acts at the level of DNA transcription and downstream protein synthesis, clinical improvements evolve gradually; steady state is reached after ~6 weeks, which is why blood tests (TSH, free T4) are typically checked 6–8 weeks after any dose change. Inter-patient dose needs vary with weight, age, pregnancy status, comorbidities, concomitant medicines, and residual thyroid function. Seemingly small changes in milligrams (or micrograms) can translate into meaningful clinical effects, so consistency of brand/formulation and timing in relation to food/supplements are key.

Across Europe and worldwide, levothyroxine is a first-line, lifelong therapy for most causes of hypothyroidism. It is also used deliberately at higher doses to suppress thyroid-stimulating hormone (TSH) in differentiated thyroid cancer follow-up and, less commonly, to help control goitre growth. When prescribed and monitored correctly, levothyroxine is safe and highly effective; most adverse effects reflect under- or overtreatment rather than allergy to the hormone itself.

Interactions

Levothyroxine’s most common issues involve absorption interference and changes in binding or metabolism. A reliable rule is to separate levothyroxine from interacting agents by several hours and to re-check labs after starting or stopping such agents.

Reduce absorption (separate by ≥4 hours)

• Minerals/supplements: Calcium, iron, magnesium, zinc.
• GI binders: cholestyramine, colesevelam, sucralfate, sevelamer, orlistat.
• Foods/drinks: coffee/espresso (if taken together), soy products, high-fibre cereals.
• Antacids/PPIs: aluminium hydroxide; PPIs may alter gastric pH and absorption—effects vary.

Alter metabolism or binding

• Enzyme inducers: rifampicin, carbamazepine, phenytoin—may increase T4 clearance; dose often needs to rise.
• Oestrogens (oral contraceptives, HRT): raise thyroxine-binding globulin → increased dose requirements.
• Sertraline and some SSRIs: may increase T4 clearance; monitor and adjust as needed.
• Amiodarone, lithium: can impair thyroid function independently; dosing needs may change.

Pharmacodynamic

• Warfarin: Restoring euthyroidism enhances anticoagulant effect; check INR after dose changes.

Patient checklist: Provide a complete list of medicines and supplements at each visit. If you start/stop an interacting agent, plan a TSH/free T4 check in ~6 weeks.

Overdose / Missed Dose

Overdose/acute excess: Symptoms may include agitation, tremor, palpitations, chest pain, diarrhoea, sweating, anxiety, and insomnia; severe cases can trigger arrhythmias, heart failure, or (rarely) thyroid storm. This is a medical emergency if large amounts were taken or if significant cardiac symptoms are present. Management is supportive: beta-blockers for adrenergic symptoms, activated charcoal if very recent ingestion, and specialist care for severe toxicity.

Chronic overtreatment: Persistently low TSH with high/upper-range free T4 over months increases risks of atrial fibrillation and osteoporosis. The remedy is dose reduction with guided monitoring; do not stop abruptly without advice.

Accidental double dose: Usually low risk—do not take extra tablets to “catch up.” Skip the next dose if advised and monitor for palpitations or insomnia. Seek advice if you have heart disease or troublesome symptoms.

Missed dose: Take when remembered if within several hours; otherwise skip and resume your usual schedule the next day. Consistency matters more than occasional slips. Never double up unless a clinician specifically instructs you to.

Always read the package leaflet and follow your clinician’s instructions. If in doubt about a dose error, speak to a pharmacist or doctor for personalised guidance.

Storage

• Store at 15–25 °C, protected from moisture, heat, and direct light. Avoid bathrooms and kitchen counters near steam or cooktops.
• Keep tablets in original blisters or a tightly closed bottle with desiccant; soft-gel/liquid formulations may have different storage needs—follow the label.
• Do not use after the expiry date. If tablets change colour or crumble, seek a fresh supply.
• Keep out of sight and reach of children. If a child ingests tablets, seek immediate medical advice.
• Dispose of unused or expired medicine via pharmacy take-back; do not flush down the toilet or bin with household waste.

Travel tip: Carry levothyroxine in hand luggage with the dispensing label. Time-zone changes are usually handled by taking the next dose at your usual local morning; perfect timing is less important than consistent daily dosing.