What is an underactive thyroid (hypothyroidism)?

Thyroid Health

Everything you need to know about hypothyroidism


Emily Condon
BSc (Hons) Biological Sciences

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What is hypothyroidism?

An underactive thyroid, also referred to as hypothyroidism, occurs when the gland does not produce enough of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Low thyroid hormone levels ultimately slow down the body’s metabolism.

What can cause hypothyroidism?

Hashimoto’s disease which is an autoimmune disorder is the most common cause of hypothyroidism. In Hashimoto’s disease, the body’s immune system mistakenly identifies the thyroid as an invader with a foreign being in the body and produces antibodies that target and destroy the thyroid gland. Destruction of the gland eventually leads to a decrease in the levels of thyroid hormones produced. In the short term, some people experience a brief period of hyperthyroidism (over-active thyroid). Like many autoimmune diseases, women have an increased risk of developing Hashimoto’s disease compared to men. 

Other causes of hypothyroidism include:

  • Thyroid surgery which may remove a part or all of the thyroid decreases the hormone levels produced.
  • Radiotherapy to the neck area which occurs when treating certain cancers damages the thyroid cells. A damaged thyroid struggles to produce the right amount of hormones that the body requires.
  • Certain medications including amiodarone, lithium, interferon alpha and interleukin-2 can sometimes affect the production of thyroid hormones. 
  • Too little iodine in the diet. The thyroid uses iodine to produce hormones. Iodine is obtained through the diet as the body can’t produce it. Low iodine levels lead to a decrease in thyroid hormones produced. High levels of iodine can also cause hypothyroidism, but in other people will lead to hyperthyroidism.

What are the symptoms of hypothyroidism?

Hypothyroidism causes a decrease in metabolic rate which causes symptoms such as:

  • Fatigue
  • Weight gain
  • Intolerance of cold temperatures
  • Low heart rate
  • Depression
  • Poor memory
  • Hair loss
  • Enlarged thyroid gland

How is hypothyroidism diagnosed and treated?

Often hypothyroidism symptoms can mimic other conditions, and because of this, hypothyroidism is often misdiagnosed. A blood test to measure thyroid hormone and antibody levels is the best way to diagnose hypothyroidism.

Symptoms that resemble hypothyroidism can be caused by a wide range of conditions so it is important to check for conditions such as diabetes, vitamin deficiencies (vitamins B12, D and folate) .and anaemia at the same time.
In the most common forms of hypothyroidism, it is the thyroid gland which is producing too little T3 and thyroxine. In these conditions, the pituitary gland is functioning normally so TSH production will be increased as the body is trying to drive up thyroxine and T3 levels. This gives the result below:

TSH: High
T3: Low or low normal
T4: Low

There are less common forms of hypothyroidism in which the pituitary gland malfunctions and produces too little TSH. So although the thyroid gland is healthy, it is understimulated by TSH and produces too little T3 and t4. This gives the result below:

TSH: Low
T3: Low or low normal
T4: Low

How is hypothyroidism treated?

There is no known way of preventing an underactive thyroid, but there are effective ways to treat and ease the symptoms. The most common treatment for hypothyroidism is the synthetic thyroid hormone levothyroxine, an oral medication capable of restoring thyroid hormone levels. Increasing hormone levels can reverse the signs and symptoms of hypothyroidism.

Learn more about thyroid disease:

What is the thyroid?

What is thyroid disease?

What are the risk factors for thyroid disease?

What is hyperthyroidism (an overactive thyroid)?

How is thyroid disease diagnosed?

Reverse T3 - what is it and do I need to test for it?

Reverse T3 - results explained

What are the symptoms of thyroid disease?

Thyroid FAQs

Medicheck your thyroid

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