Underactive thyroid and weight gain

This content was last reviewed in September 2019 by Jason Webb, a registered nurse in our Health at Hand team.

Hypothyroidism – also known as an underactive thyroid – is where your thyroid gland doesn’t produce enough of the hormones that control your metabolism. This changes the way your body handles fat, which makes you more prone to weight gain. The replacement hormone is called levothyroxine.

Most people respond well to replacement hormone medication, but is also worth undertaking regular exercise and eating a healthy diet (whether you have an underactive thyroid or not).

Hypothyroidism causes

You cannot prevent an underactive thyroid, but there are two main causes:

• The immune system attacks the thyroid. This damages the thyroid and stops it producing thyroxine (T4), the hormone needed to regulate your metabolism. This is the most common cause of hypothyroidism.
• Damage to the thyroid. This usually happens when you undergo surgery or other treatments for issues with your thyroid, such as thyroid cancer or an overactive thyroid.

There are other, less common causes of hypothyroidism. These include:

• An iodine deficiency
• When the thyroid gland doesn’t develop properly in the womb
• When there is a problem with your pituitary gland, which regulates the thyroid
• Some viruses and medications have also been linked to hypothyroidism.

Hypothyroidism symptoms

The symptoms of an underactive thyroid tend to build up slowly and are quite general so it can be hard to pinpoint the condition. These include:

• Tiredness
• Weight gain
• Constipation
• Low mood or depression
• Feelings of pain, numbness or tingling in your hand and fingers
• Sensitivity to the cold
• Dry skin and hair
• Muscles that ache, feel weak or cramp
• Reduced libido
• Irregular or heavy periods.

Other symptoms of hypothyroidism can relate more to your age. For example, older people tend to become forgetful or depressed, children may not grow and develop as quickly as others, whereas teenaged may start puberty early.

If you do not treat your underactive thyroid, you may also experience:

• Low heart rate
• Reduced/lost hearing
• Low iron levels
• A lower tone of your voice, which could also become hoarse
• Thinning eyebrows
• A puffy face.

If you have any of these symptoms you must see your doctor as soon as possible.

Hypothyroidism test

It's important to get hypothyroidism diagnosed as soon as possible as the health implications can be serious.

A thyroid function test is used to accurately assess if you have an underactive thyroid. This is a blood test that measures thyroxine (T4) and thyroid-stimulating hormone (TSH) levels.

If you have a lot of TSH but not much T4 you could have an underactive thyroid.

If you have a lot of TSH but normal T4 levels you could be as risk of developing an underactive thyroid. Doctors will give you regular tests to monitor this.

Hypothyroidism treatment

If you only have mild symptoms of hypothyroidism and your levels aren’t too bad, doctors may not prescribe any treatment but will monitor your situation (unless you are pregnant or looking to get pregnant).

Hypothyroidism is usually treated with a medication called levothyroxine, which replaces the hormone your gland isn’t making.

There is usually a period of trial and error when you start taking levothyroxine as doctors adjust your dose to get it right. Once this is sorted you have an annual blood test to check your T4 and TSH levels.

There aren’t usually any side effects of taking levothyroxine. Sometimes you may experience diarrhoea, vomiting, headaches, chest pains and sweating, particularly if the dose it too high.

Sometimes you also have to be careful mixing levothyroxine with other medications. For example, there are two well recognised connections between thyroid function and statins:

• An underactive thyroid can increase cholesterol levels in the body so if your thyroid treatment is successful there may be no need to take statins.
• One of the side effects of statins is myopathy – an inflammation of your muscles, causing muscle pain, which can sometimes be severe. Having an underactive thyroid can increase the risk of statin-induced myopathy. You don’t need to avoid statins completely if you’re taking thyroxine, but you should report promptly any unexplained muscle pain, tenderness or weakness and should see your GP if the aching is significant or doesn’t settle.

Promoting weight loss, healthy eating and exercise

Once you have commenced levothyroxine and the blood plasma levels have been checked and they are at the right therapeutic level – exercise and healthy eating should not be a barrier to weight loss. 

The current government guidelines for healthy eating can be found on the NHS website.  If you would like to lose weight, then you can use the free 12-week diet and exercise plan to help you lose weight using the NHS weight loss plan. On this site, you will also find an activity plan for beginners, whereby you will be using strength, running and flexibility workouts, so that you can learn new habits for developing an active lifestyle and help promote your weight loss.

Hypothyroidism and pregnancy

If left untreated, an underactive thyroid can result in some complications occurring during pregnancy. For the mother, these include:

• Pre-eclampsia, and all the associated issues this causes, such as high blood pressure
• Anaemia
• Bleeding after birth.

For the baby:

• An underactive thyroid
• Birth defects
• Development issues
• Stillbirth or miscarriage.

You must speak to your doctor immediately if you have an underactive thyroid and are pregnant or trying for a baby as these can be avoided if you get specialist treatment.

Further Reading

Hypothyroidism - NHS factsheet

Hyperthyroidism - NHS factsheet

How to lose weight well - AXA Health

Steps to becoming active your way - AXA Health

Diet and nutrition hub - AXA Health

Exercise and fitness hub - AXA Health