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Underactive thyroid (hypothyroidism)

Key points

  • Hypothyroidism means you have an underactive thyroid. This is caused by your thyroid gland not producing enough thyroid hormones.
  • The symptoms of underactive thyroid include feeling extremely tired, gaining weight, feeling the cold, thinning hair, weak muscles and an enlarged thyroid gland.
  • Underactive thyroid is treated with thyroid hormone replacement medicine called levothyroxine. When you’re first diagnosed, it may take a while for your symptoms to ease until your GP finds the right dosage of medicine for you.
  • Thyroid function tests can help manage your thyroid disorder and ensure you’re receiving the right dosage of medication.

This section contains answers to frequently asked questions about this topic. Questions have been suggested by health professionals, website feedback and requests via email.

What blood tests are used to diagnose an underactive thyroid?


To check your thyroid function, your GP will take a sample of your blood and check your levels of thyroid-stimulating hormone (TSH). Your level of thyroxine (T4) may also be measured.


TSH is produced by a part of your brain called the pituitary gland. It stimulates your thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Together, these hormones control how quickly your body uses its energy stores (metabolism) and how sensitive your body is to other hormones.

To check your thyroid function, your GP will take a sample of your blood and measure the levels of TSH. If this is high, the level of thyroxine in your blood may also be measured.

If the results show you have high levels of TSH with low levels of thyroxine, it means you have an underactive thyroid.

How long will it take for me to feel better once I've started taking levothyroxine for underactive thyroid?


You should expect a gradual improvement, feeling back to normal about four to six months after starting levothyroxine treatment. Once your thyroid hormone levels are back to normal, you may start to feel better fairly quickly, but this varies from person to person.


Treatment with levothyroxine aims to return your levels of thyroid hormones to normal.
If you’re over 18, your doctor may start you on a levothyroxine dose of 50 to 100 micrograms (μg) once a day. You will be monitored regularly and your dose will be adjusted in steps of 25 to 50μg every three to four weeks, depending on how to you respond to it.

Your symptoms will begin to improve as the hormone levels in your bloodstream return to normal. The length of time this takes will depend on how severe your underactive thyroid was when you were diagnosed and the dose you started on. It can sometimes take several months after your thyroid levels are stable before you start feeling better, so don’t worry if your symptoms don’t ease straight away.

You will be closely monitored by your doctor, especially in the first six months of treatment. It’s important to let your doctor know if you don’t feel any better or if you have any side-effects.

Side-effects to look out for include diarrhoea, vomiting, chest pains, irregular heartbeat, flushing, weight loss, headaches and muscle cramps. Having side-effects may be a sign that the dose of levothyroxine you’re taking is too high, so it’s important to tell your doctor so that your dose can be reduced.

Is it safe to take levothyroxine for underactive thyroid during pregnancy?


Yes, it's safe to take levothyroxine during pregnancy. You will need to be closely monitored during your pregnancy to ensure your thyroid hormones are at a healthy level.


During pregnancy, it's important that you have enough thyroid hormones in your bloodstream. This is because in the first trimester (first 12 weeks) of pregnancy, thyroid hormones help to develop your baby’s brain and nervous system.

If you have an underactive thyroid, see your GP as soon as you think you’re pregnant. They will usually increase your dose of levothyroxine to ensure you have enough thyroid hormones for you and your baby. How much this is increased by will depend on the current dose you're taking and the results of your blood test.

Your thyroid hormone levels will be regularly checked during your pregnancy. If you were diagnosed with an underactive thyroid very recently before becoming pregnant, your GP may refer you to an endocrinologist. This is a doctor who specialises in conditions of the endocrine system. They will keep a close eye on you throughout your pregnancy.

If you have any questions or concerns about underactive thyroid and pregnancy, speak to your GP or endocrinologist.

Reviewed by Alice Rossiter, Bupa Health Information Team, May 2014.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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