Levothyroxine

Key facts

• Levothyroxine is synthetic thyroxine (T4), the hormone your thyroid should make; it treats an underactive thyroid (hypothyroidism) and related conditions.
• Take it on an empty stomach at the same time every day, ideally 30 to 60 minutes before breakfast, and swallow it with water alone.
• Separate it from iron, calcium and antacids by at least 4 hours: these reduce how much you absorb. Taking too much causes over-replacement, which raises the risk of an irregular heartbeat (atrial fibrillation), especially in older adults.
• Your dose is set and changed using TSH blood tests, not by how you feel; never adjust it yourself.

What levothyroxine treats

Levothyroxine treats hypothyroidism, an underactive thyroid that causes fatigue, weight gain, constipation and feeling cold. It replaces the hormone after thyroid surgery or radioactive iodine treatment, manages a goitre (enlarged thyroid), and suppresses TSH after treatment for certain thyroid cancers. Babies born with an underactive thyroid also need it, usually for life.

How levothyroxine works

Levothyroxine is identical to your body's own T4 hormone. Once absorbed, tissues convert some of it into T3, the more active form, which binds receptors inside cells and sets the pace of your metabolism, heart rate and body temperature.

Before you take it

• Tell your prescriber about heart disease, untreated adrenal insufficiency, or a previous allergic reaction to levothyroxine.
• Pregnancy usually requires a higher dose, since the baby's brain development depends on adequate thyroid hormone; TSH is checked more often and treatment continues throughout.
• Iron, calcium supplements, antacids, and some ulcer medicines block absorption if taken too close together. Warfarin's effect can change once your thyroid level shifts.
• If you switch brands or generic versions, tell your prescriber; strengths can vary slightly and a repeat TSH check is sensible.

Side effects

At the correct dose, side effects are uncommon. Symptoms usually mean the dose is too high: nervousness, tremor, sweating, increased appetite with weight loss, and trouble sleeping.

Stop and seek urgent medical care for any of these:

• A fast, pounding or irregular heartbeat.
• Chest pain or tightness.
• Severe headache, agitation, or confusion.
• Signs of an allergic reaction: rash, swelling of the face or throat, difficulty breathing.

Safety essentials

• Over-replacement is the main danger with this drug: it strains the heart and raises the risk of atrial fibrillation, particularly in older adults. Doses are started low and raised gradually under TSH monitoring, especially later in life.
• Keep the empty-stomach, same-time-daily routine consistent. Taking it with food, or too close to iron, calcium or antacids, can undermine an otherwise correct dose.
• Treatment is usually lifelong; do not stop without medical advice, since symptoms of an underactive thyroid can return gradually and go unnoticed.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.