Levothyroxine for Hypothyroidism: Uses and Safety Tips

6 min read · Medicines · By the Asthra Health Care team

What levothyroxine is used for

Levothyroxine, also called thyroxine, is a prescription medicine used to treat hypothyroidism, a condition where the thyroid gland does not make enough hormone. Hypothyroidism is common in India, particularly in women, and causes tiredness, weight gain, feeling cold, constipation, dry skin, hair fall, low mood and irregular periods.

Doctors also use levothyroxine after thyroid surgery or radioiodine treatment, for some thyroid swellings, and in pregnancy when thyroid levels are low, because thyroid hormone is important for the baby's brain development. Babies found to have low thyroid on newborn screening need prompt treatment.

• Hypothyroidism, including Hashimoto's thyroiditis
• After thyroid surgery or radioiodine treatment
• Low thyroid in pregnancy
• Congenital hypothyroidism in newborns

How levothyroxine for hypothyroidism works

Levothyroxine is identical to T4, the main hormone made by the thyroid gland. Once absorbed, the body converts it into the active form, T3, which controls how fast the body uses energy. Replacing the missing hormone brings energy levels, metabolism, heart rate and many other body functions back to normal.

It works slowly. Most people start feeling better within a few weeks, but it takes around six weeks for blood levels to settle, which is why doctors usually wait that long before rechecking tests and adjusting the amount. Do not judge whether it is working by how you feel in the first week or two; the blood test is the best guide.

Levothyroxine for hypothyroidism side effects: common and serious

When the amount is right, levothyroxine usually causes no side effects, because it simply replaces a natural hormone. Most side effects come from taking too much, which produces signs of an overactive thyroid. Too little, on the other hand, leaves the original symptoms of low thyroid in place. Regular blood tests help your doctor find the right amount for you.

Signs of too much include a fast or irregular heartbeat, palpitations, shaking, sweating, feeling hot, anxiety, difficulty sleeping, diarrhoea and weight loss. Over time, too much hormone can weaken bones and strain the heart, especially in older people. Some people notice mild temporary hair thinning when starting. Rare allergic reactions to tablet ingredients can occur.

Who should take extra care with levothyroxine for hypothyroidism

Elderly people and those with heart disease, especially angina or irregular heartbeat, are usually started on a lower amount and increased slowly, as a sudden rise in thyroid hormone can strain the heart. People with diabetes may notice changes in sugar levels. Those with untreated adrenal gland problems need these treated first.

In pregnancy, thyroid hormone needs usually rise, so women on levothyroxine should tell their doctor as soon as they are pregnant for early testing and adjustment. It is safe in breastfeeding. Children need regular review as they grow. Levothyroxine is only on a doctor's prescription; do not change or stop it on your own.

Levothyroxine for hypothyroidism interactions to know

Many substances reduce how well levothyroxine is absorbed. These include calcium and iron tablets, antacids, some acid-reducing medicines, multivitamins with minerals, soya products, high-fibre cereals, and coffee or milk taken at the same time. Keeping a gap of a few hours between levothyroxine and these is usually advised.

Some epilepsy medicines, rifampicin and oestrogen-containing pills can change how much levothyroxine you need. Levothyroxine can increase the effect of warfarin and change the needs of diabetes medicines. Tell your doctor about all medicines and supplements, and remind them when starting any new one.

Practical tips for using levothyroxine for hypothyroidism

Take levothyroxine exactly as prescribed, usually first thing in the morning on an empty stomach with plain water, and wait before having tea, coffee, milk or breakfast. Your doctor or pharmacist will tell you how long to wait. Doing it the same way every day keeps levels steady.

Stick to the same brand or source if possible, as switching can slightly change levels; tell your doctor if it changes. Store tablets away from heat and moisture. Get your TSH tested as your doctor advises, usually a few weeks after any change and then periodically. If you miss a tablet, ask your pharmacist what to do.

When to talk to your doctor about levothyroxine for hypothyroidism

See your doctor if tiredness and other symptoms continue, or if you notice palpitations, shaking, weight loss or poor sleep. Seek urgent care for chest pain or a very fast, irregular heartbeat. Tell your doctor right away if you become pregnant. Ask your doctor or pharmacist whenever you have questions about your thyroid medicine.

This article is general health information, not medical advice. Do not start, stop or combine medicines on your own — always follow your doctor's prescription and ask your pharmacist if unsure.