Underactive thyroid (hypothyroidism)


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 assess your thyroid function, your GP will take a sample of your blood and check your levels of thyroid-stimulating hormone, and the hormones released by your thyroid gland (thyroxine and triiodothyronine).


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

To assess your thyroid function, your GP will take a sample of your blood and measure the levels of thyroid-stimulating hormone, thyroxine and triiodothyronine in your blood.

If the results show you have high levels of thyroid-stimulating hormone 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 according to your response. The usual maintenance dose is 75 to 150μg once a day.

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 on the dose you start on. It can sometimes take several months after your thyroid levels are stable before you start feeling better.

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 hormone levels 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, you must see your GP as soon as you think you’re pregnant. Your doctor will usually increase your dose of levothyroxine to ensure you have enough thyroid hormones for you and your baby. The increase 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 have only been diagnosed with underactive thyroid very recently before becoming pregnant, or you have subclinical hypothyroidism (which means you don't have any symptoms but your thyroid hormone levels are disrupted), you may need to have blood tests more frequently. Your GP may refer you to an endocrinologist (a doctor who specialises in the treatment of conditions relating to your endocrine system) for regular check-ups throughout your pregnancy.

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


Produced by Krysta Munford, Bupa Health Information Team, May 2012.

For our main content on this topic, see Information.

For sources and links to further information, see Resources.

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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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