An underactive thyroid is what happens when your thyroid gland doesn't produce enough hormones. It's also referred to as a hypothyroid. Symptoms include fatigue, weight gain, and difficulty concentrating. An easy way to track what's going on with your thyroid gland is by taking regular blood tests.

What is an underactive thyroid?

Your thyroid gland makes hormones that help control your metabolism. If you don’t make enough of these hormones you have an underactive thyroid, also known as hypothyroid. You can measure your thyroid hormone levels with a special thyroid function blood test.

Underactive thyroid symptoms

The lack of thyroid hormones causes a ‘slowing down’ of many of the body’s functions, causing the following symptoms:

  • Fatigue
  • Weight gain
  • Irregular periods – usually heavy bleeding
  • Difficulty concentrating or ‘slowing’ of thought
  • Dry skin
  • Brittle hair or hair loss (including the hair on the outer third of the eyebrow)
  • Intolerance to the cold
  • Low mood
  • Constipation
  • Hoarse voice
  • Muscle aches or cramps
  • Weakness

Causes of underactive thyroid

There can be a few reasons for the development of hypothyroidism, such as:

Autoimmune thyroiditis

This is the most common cause of hypothyroidism in the UK. Normally, the body’s immune system makes antibodies to attack foreign bacteria and viruses. In autoimmune thyroiditis, the body produces antibodies that act against the thyroid gland, damaging it.

The gland stops functioning normally and your levels of the thyroid hormone thyroxine drop. This leads to hypothyroidism.

Autoimmune thyroiditis is associated with other autoimmune conditions, including Coeliac disease, pernicious anaemia, vitiligo, Addison’s disease, and Type 1 diabetes.

Other related thyroid diseases are:

Hashimoto’s disease - autoimmune thyroiditis with a goitre (an abnormal swelling of the neck)

Atrophic autoimmune hypothyroidism - this is a very rare thyroid disease and is basically autoimmune hypothyroidism, but without a goitre.

Surgery or treatment to the thyroid gland

Underactive thyroid hypothyroidism can also occur as a side effect of treatment to a previously overactive thyroid gland.

Postpartum thyroiditis

In rare cases, your thyroid gland can become inflamed after giving birth. This can cause a change in the levels of the thyroid hormones you produce. You might then suffer from hypothyroidism.

Postpartum thyroiditis is usually not permanent, and your gland goes back to normal after 12-18 months. However, in a few, very uncommon instances you might develop a permanent problem with your thyroid gland.

Iodine deficiency

The body needs iodine to make the thyroid hormone thyroxine. A lack of iodine in the diet can cause hypothyroidism.

Iodine deficiency is the most common cause of hypothyroidism worldwide. However, it's not common in the UK.


Some medications used to treat other illnesses can cause hypothyroidism as a side effect.

The medication most commonly associated with thyroid disease is amiodarone. This is a drug used in the treatment of heart abnormalities.

Blood tests and hypothyroidism

You can test your thyroid in a few ways. You can go to your GP and get a blood test that way. You can also order an at-home blood test. Both of these types of blood tests will measure your TSH levels. The third way is to get an advanced thyroid test, which will give you a much more comprehensive overview of your thyroid.

What are the treatments for an underactive thyroid?

The most common treatment for an underactive thyroid is taking tablets of levothyroxine – a hormone replacement used to raise your thyroxine levels.

What is TSH and how is it related to an underactive thyroid?

Sometimes your body won’t produce enough of the thyroid hormones. When this happens, your body sends a signal to the pituitary gland, which in turn makes Thyroid Stimulating Hormone (TSH). TSH stimulates the thyroid gland to increase the production of thyroid hormones T3 and T4.

If there is not enough thyroid hormone present, a signal is sent to the pituitary resulting in the secretion of TSH that then stimulates the thyroid to increase the production of T3 and T4.

This means that your TSH levels will be high when our thyroid hormone levels are too low. So a blood test showing high TSH is a sign of hypothyroidism.

Note that problems with the thyroid are most commonly caused by issues in the thyroid gland itself, rather than the hypothalamus or pituitary glands.

Subclinical hypothyroidism

In some situations, your TSH levels can be above the upper end of the normal range (0.5mU/L - 4.5mU/L) but your T4 and T3 levels are normal, rather than lowered. This is called Subclinical Hypothyroidism (SCH).

Is subclinical hypothyroidism serious? Will I need treatment?

The majority of patients with SCH have no symptoms, and UK guidelines state that most people do not need treatment.

It's still important to note that 20-50% of people with SCH will go on to develop ‘clinical’ hypothyroidism – raised TSH and low free T4. The risk of this happening is increased further if a person also has Anti-thyroid Peroxidase(TPOAb). This is a thyroid autoimmune disease that results from the abnormal production of TPO antibodies by the immune system.

Current UK guidelines suggest treating a person under the age of 70 with levothyroxine if their TSH is greater than 10mU/L even if they do not have any symptoms.

If the TSH is between 4 and 10mU/L, treatment should be considered if a person has symptoms of hypothyroidism.

The symptoms of hypothyroidism can be very non-specific e.g. feeling tired all of the time. It may be inaccurate to label SCH as the cause, and the true cause may be overlooked as a result.

This is why it's important for your doctor to rule out other causes of your symptoms before ascribing them to subclinical hypothyroidism. 


TSH - Thyroid Stimulating Hormone

T3 - Triiodothyronine

T4 - Thyroxine

TPOAb: Anti-thyroid Peroxidase. A thyroid autoimmune disease that results from the abnormal production of TPO antibodies by the immune system

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