Levothyroxine

Thyroid Hormone Replacement·Endocrinology - Thyroid & Bone

Primary hypothyroidism, Hashimoto's thyroiditis, post-thyroidectomy

PBSTHYROIDOroxine, Eutroxsig, EltroxinTablets (50 mcg, 75 mcg, 100…S4+1

Max Dose

200 mcg/day

Half-life

7 DAYS

Empty Stomach

MANDATORY

Arrhythmia

TOXICITY

Rapid Summary

Overview

Synthetic thyroxine (T4). The gold standard replacement hormone for hypothyroidism. Narrow therapeutic index.

Maximum Dose

Titrated strictly via TSH levels. Usually 50-200 mcg/day.

Clinical Focus

Absorption is highly erratic. Must be taken strictly on an empty stomach with water only.

Bottom Line

Levothyroxine is the standard thyroid hormone replacement for hypothyroidism, but has extensive drug and food interactions (take fasting) and requires careful dose titration with regular TSH monitoring.

Formulation & Access

Brand Names

Oroxine, Eutroxsig, Eltroxin

Oral Routes

Tablets (50 mcg, 75 mcg, 100 mcg, 200 mcg). Storage is brand-specific: Oroxine/Eutroxsig require refrigeration; Eltroxin storage differs.

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Primary hypothyroidism, Hashimoto's thyroiditis, post-thyroidectomy.

Clinical Place

Lifelong mandatory therapy for absent/failing thyroid gland.

Quick reference

Class & Role

Thyroid Hormone Replacement — Synthetic thyroxine (T4).

Route / Formulation

Tablets (50 mcg, 75 mcg, 100 mcg, 200 mcg). MUST be kept refrigerated in WA Health (unless Eltroxin brand). (Oroxine, Eutroxsig, Eltroxin)

Usual Dose & Max

PO 1.6 mcg/kg OD (typically 100-150 mcg). Titrated strictly via TSH levels. Usually 50-200 mcg/day.

Key Indication Doses

Hypothyroidism (Young/Healthy): PO 1.6 mcg/kg OD (typically 100-150 mcg). Elderly / Ischemic Heart Disease: Start at PO 12.5 - 25 mcg OD. Titrate up glacially (every 4-6 weeks) to prevent acute myocardial infarction.

Best Uses

Avoid / Cautions

Uncorrected adrenal insufficiency (Addison's). Starting T4 will hyper-accelerate metabolism and precipitate a fatal Addisonian crisis. Treat the adrenals first. Pregnancy Category A. Demands are HIGH in pregnancy. Dose must often be increased by 30-50% the moment pregnancy is confirmed to ensure fetal brain development.

Key Risks

Cardiovascular: Iatrogenic hyperthyroidism (overdose) causes AF, tachycardia, angina, and myocardial infarction. Musculoskeletal: Chronic mild over-replacement suppresses TSH and accelerates bone resorption, causing severe osteoporosis over decades. Neurological: Tremor, anxiety, insomnia (signs of overdose).

Key Interactions

Calcium, Iron, Antacids, and Coffee bind thyroxine in the gut, completely destroying absorption. Must be separated by at least 4 hours.

Monitoring

Monitor TSH every 6 weeks during titration. Do NOT check levels 3 days after a dose change; it takes 6 weeks for the HPA axis to stabilize. Check HR for tachyarrhythmias.

Clinical Pearl

If a patient says their dose 'keeps having to be changed', they are taking it with coffee or breakfast. Counsel them to take it immediately upon waking, with water only, and wait 30-60 mins before eating or drinking coffee.

Access

Oroxine, Eutroxsig, EltroxinTablets (50 mcg, 75 mcg, 100…

Brand: Oroxine, Eutroxsig, Eltroxin
Formulation: Tablets (50 mcg, 75 mcg, 100 mcg, 200 mcg)