[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Thyroid hormones](https://durgical.com/en/pharmacology/classes/thyroid-hormones/) › Levothyroxine

[Pharmacology](https://durgical.com/en/pharmacology/) · [Thyroid hormones](https://durgical.com/en/pharmacology/classes/thyroid-hormones/)

# Levothyroxine

Synthetic thyroid hormone (T4) taken daily to replace what an underactive thyroid no longer makes. Adjusted with blood TSH.

Updated 2026-11-01

- Class: [Thyroid hormones](https://durgical.com/en/pharmacology/classes/thyroid-hormones/)
- Receptor: Thyroid hormone receptor β (and α)
- ATC code: H03AA01
- WHO Model List of Essential Medicines

Also called: L-thyroxine, T4

◎**Thyroid hormone receptor β (and α)**Levothyroxine

ⓘ The figure shows thyroxine (T4, levothyroxine itself) bound to the ligand-binding domain of human thyroid hormone receptor β; in the body most of the effect comes from T3 made from it.

**Target in 3D:** Thyroid hormone receptor β (and α). PDB structure 1Y0X (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/1Y0X) · Drag to rotate · pinch to zoom

## Mechanism of action

Levothyroxine is identical to natural thyroxine (T4). In tissues, deiodinases remove one iodine and turn it into T3, which binds nuclear thyroid hormone receptors and changes transcription of genes that set metabolic rate, heat production, heart rate, growth and brain development. Because T4 has a long half-life and is converted steadily, once-daily dosing gives stable levels.

- Levothyroxine (T4) circulates bound to plasma proteins
- Deiodinases in tissues turn T4 into active T3
- T3 binds nuclear thyroid hormone receptors
- Gene transcription changes: metabolism and heart rate rise

## Pharmacokinetics

**Absorption**
Absorbed in the small intestine; food, calcium, iron and some drugs reduce absorption

**Distribution**
Almost entirely bound to plasma proteins

**Metabolism**
Deiodinated in tissues to T3 (active) and reverse T3 (inactive)

**Half-life**
Long, longer than T3, so changes in dose take weeks to show in TSH

## Indications

- Primary, secondary or tertiary hypothyroidism, congenital or acquired [Hypothyroidism →](https://durgical.com/en/conditions/hypothyroidism/)
- TSH suppression after surgery and radioactive iodine for differentiated thyroid cancer [Thyroid cancer (papillary) →](https://durgical.com/en/conditions/thyroid-cancer/)

## Adverse effects

### Common

- Symptoms of excess hormone: palpitations, tremor, insomnia, heat intolerance, weight loss
- Diarrhoea
- Partial hair loss in the first months
- Menstrual changes

### Serious (rare, important)

- Angina, arrhythmias (atrial fibrillation) and myocardial infarction, especially in the elderly or with heart disease
- Loss of bone mineral density with long-term overtreatment
- Adrenal crisis if adrenal insufficiency is not corrected first

## Contraindications

- Uncorrected adrenal insufficiency
- Treatment of obesity or weight loss (boxed warning)

## Key interactions

**Calcium carbonate, iron salts, phosphate binders (sevelamer, lanthanum)**
Bind levothyroxine and reduce its absorption; doses are separated by several hours

**Bile acid sequestrants (cholestyramine, colesevelam) and ion-exchange resins**
Lower absorption; levothyroxine is given hours before them

**Proton pump inhibitors and antacids**
May lower absorption

**Warfarin**
Stronger anticoagulant effect as thyroid function normalises

**Insulin and oral antidiabetic drugs**
Glucose control may worsen; requirements may rise

**Digoxin**
Digoxin levels and effect may fall

## What is monitored

- TSH (free T4 in central hypothyroidism)
- Heart rate and symptoms of excess hormone
- Thyroid tests early in pregnancy and through it
- Growth and development in children

## Administration and safety

- Given once daily on an empty stomach, before breakfast with water
- Separated by several hours from calcium, iron, antacids and binding resins
- Narrow therapeutic index: switching products can change levels, so TSH is rechecked
- Biotin supplements can distort thyroid test results

## Nursing care

- Teach a fixed daily routine: same time, empty stomach, with water
- Check heart rate; report palpitations, chest pain or tremor
- Explain that it is usually lifelong and that improvement takes weeks
- Ask women to report pregnancy early: needs usually rise
- Review other drugs and supplements taken at the same time of day

## Pregnancy and breastfeeding

Essential in pregnancy when hypothyroidism exists: needs usually rise, so thyroid tests are checked often; untreated hypothyroidism harms the baby.

## Memory aid

LEVO: Lifelong, Empty stomach, Very long half-life, Obey the TSH.

## Test yourself

Think of the answer, then tap the question to see it.

## Why is levothyroxine given on an empty stomach and apart from calcium or iron?

Food, calcium and iron bind it or reduce its absorption, making levels lower and less predictable.

## Why are TSH levels rechecked only weeks after a levothyroxine dose change?

T4 has a long half-life, so a new steady state and the TSH response take weeks to settle.

## An older patient with coronary disease starts levothyroxine. What is the main risk?

Angina, arrhythmia or infarction from the rise in heart rate and oxygen demand; treatment starts low and rises slowly.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/levothyroxine/)

## Related drugs

- [Methimazole](https://durgical.com/en/pharmacology/methimazole/)

## Related conditions

- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)

[← Back to the class: Thyroid hormones](https://durgical.com/en/pharmacology/classes/thyroid-hormones/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Levothyroxine: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682461.html) (MedlinePlus (U.S. National Library of Medicine), 2019); [DailyMed - LEVOTHYROXINE SODIUM tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0fa2e2f5-401b-45c1-b6a5-d02f6faf509d) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=H03AA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/pharmacology/levothyroxine/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/levothyroxine/