Levothyroxine: Uses, Side Effects & Safety

What it is and what it's used for

Levothyroxine is a man-made version of thyroxine (T4), the main hormone normally produced by the thyroid gland in the neck. It belongs to the group of thyroid hormone medicines. Doctors prescribe it when the thyroid gland cannot make enough hormone on its own, a condition called hypothyroidism. It is also used for some types of goitre (swelling of the thyroid gland), after thyroid gland surgery, and after radioiodine treatment for thyroid problems.

Levothyroxine is a prescription-only medicine in India (a Schedule H drug), so you need a doctor's prescription to buy it. Because the dose must be matched to your own hormone levels, treatment usually starts with blood tests and the dose is adjusted over a few weeks or months based on follow-up thyroid tests.

How it works (simple language)

The thyroid hormone controls how fast the body's engine runs — things like energy use, heart rate, digestion, and body temperature. In hypothyroidism this engine runs too slowly, causing tiredness, weight gain, feeling cold, and constipation. Levothyroxine simply replaces the missing hormone, giving the body the thyroxine it would have made itself.

Once swallowed, the medicine is absorbed in the intestine and the body converts it into its active form (T3) in the liver and other tissues. It does not stimulate or "fix" the thyroid gland itself — it acts as a replacement, which is why many people take it long-term or even lifelong.

Dosage forms available in India (general info only)

In India levothyroxine is most commonly available as plain tablets, in strengths such as 25, 50, 75, 100, 125 and 150 mcg. Well-known Indian brand names include Thyrox and Thyronorm, though brands may vary by pharmacy. It is also sometimes prescribed in combination preparations only when the doctor specifically advises them.

Never decide the dose yourself from this article. Always follow your doctor's prescription or the directions on the pack, and check with your pharmacist if you are unsure — especially for children, where the dose depends on the child's weight and age.

Side effects

Common: Hair loss (usually temporary and reversible once the dose settles), weight changes, increased appetite, sweating or feeling warm, nervousness, restlessness, trouble sleeping, headache, diarrhoea, and a fast or pounding heartbeat. These often appear when the dose is slightly too high and usually improve after the doctor adjusts it.

Serious — seek medical care: Chest pain or pressure, a very fast, irregular, or pounding heartbeat, shortness of breath, severe headache, leg cramps, or symptoms of too much thyroid hormone such as marked trembling, extreme nervousness, or confusion. Stop the medicine and seek emergency care if you develop signs of an allergic reaction such as rash, swelling of the face or throat, or difficulty breathing.

Precautions and warnings

• Empty stomach dosing: Take levothyroxine on an empty stomach, 30 to 60 minutes before breakfast (or 3 to 4 hours after your last meal), with a glass of water. Food, tea, coffee, and milk can reduce how much medicine is absorbed.
• Same time daily: Take it at the same time each day to keep hormone levels steady in the body.
• Do not switch brands casually: Different brands may release the hormone slightly differently. If your pharmacy changes the brand, tell your doctor — a repeat thyroid blood test may be needed.
• Do not stop suddenly: Hypothyroidism is usually a long-term condition. Stopping the medicine can bring symptoms back and slow down body functions again. Any change should be guided by your doctor.
• Heart patients need extra care: Older adults and people with heart disease usually start on a lower dose, because too much thyroid hormone can strain the heart.
• Diabetes and bone health: Tell your doctor if you have diabetes (blood sugar control may shift when treatment starts) or osteoporosis, as dose adjustments may be needed.
• Surgery and illness: Inform your doctor or anaesthetist that you take levothyroxine before any surgery, and discuss what to do during serious illness or fever.

Interactions (general)

Levothyroxine should be taken at least 4 hours apart from calcium or iron supplements, multivitamins containing these minerals, and antacids, because they can bind the medicine in the gut and reduce absorption. Soy products and high-fibre foods may also reduce absorption if eaten close to the dose. It can increase the effect of blood-thinning medicines like warfarin, may change the dose needed for diabetes medicines, and its effect can be altered by seizure medicines (phenytoin, carbamazepine), oestrogen medicines, and some antidepressants. Tell your doctor about ALL medicines you take, including herbal and ayurvedic products, and avoid alcohol around dosing time.

Pregnancy and breastfeeding (general guidance)

Hypothyroidism during pregnancy needs careful control, because both mother and baby depend on adequate thyroid hormone. Women who are already on levothyroxine often need a higher dose during pregnancy, and the dose is usually checked with blood tests once pregnancy is confirmed. Do not stop the medicine if you become pregnant — consult your doctor first, as stopping can be harmful. Small amounts pass into breast milk but breastfeeding is generally considered safe while on levothyroxine, with the doctor's guidance.

Storage

Store tablets at room temperature, away from heat, moisture, and direct sunlight — do not keep them in the bathroom. Keep the tablets in their original pack until use, keep them out of reach of children, and do not use them after the expiry date printed on the pack.

When to see a doctor

See a doctor if: your hypothyroid symptoms (tiredness, weight gain, feeling cold, constipation) do not improve after several weeks of treatment; you develop a fast or irregular heartbeat, chest pain, shortness of breath, or severe trembling; you become pregnant or plan to; you start or stop any other medicine, supplement, or ayurvedic product; or your pharmacy changes the brand. Seek emergency care immediately for chest pain, fainting, or signs of a severe allergic reaction.

FAQs

1. How long does levothyroxine take to work?

Most people start feeling a little better within 1 to 2 weeks, with energy and mood improving first. However, it usually takes 4 to 6 weeks for the full effect, and your doctor will typically recheck your thyroid blood levels (TSH) after about 6 to 8 weeks before adjusting the dose.

2. Can I take levothyroxine with tea or coffee?

No — tea, coffee, milk, and food can all reduce how much of the medicine your body absorbs. Take the tablet on an empty stomach with water, and wait 30 to 60 minutes before having breakfast, tea, or coffee.

3. What should I do if I miss a dose?

Take it as soon as you remember, unless it is almost time for your next dose. In that case skip the missed one and continue as normal. Never take a double dose to make up for a missed one — if you miss doses often, tell your doctor.

4. Can I stop levothyroxine once my thyroid levels become normal?

Usually no. Normal blood levels generally mean the medicine is working, not that the underlying thyroid problem is cured. Most people with hypothyroidism need lifelong treatment, and stopping can bring symptoms back. Any decision to stop or change the dose must be made with your doctor, guided by blood tests.