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Hypothyroidism (underactive thyroid)

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 4 minutes read

Your thyroid gland is a small gland in the front of your neck. It is involved in a chain reaction, where your brain releases a hormone, which prompts the thyroid gland to release a hormone (called thyroid-stimulating hormone or TSH), which then produces thyroxine, also called free T4. This is the active biochemical.

Hypothyroidism means that your thyroid gland is underactive, or not making enough thyroid hormone. You experience symptoms of a slowed metabolism, which affects how you use and store energy.

This can affect the body in many ways:

  • weight gain
  • feeling cold all the time
  • feeling tired
  • feeling sluggish
  • needing to sleep more
  • dry skin
  • hair loss
  • slower in your thinking
  • less able to concentrate
  • feeling a bit depressed
  • having a slower or weaker pulse

Hypothyroidism is approximately five times more common in women than in men and in the US about 1 in 100 people have it (although some research indicates that it may be has high as 1 in 10). It is most common in adult women and becomes more common with increasing age, but it can occur at any age and can affect anyone. Fortunately, it is easily reversed once it’s been identified and treatment started, although it can take a month or two for this to take full effect.

How do I know if I have it?

If you are concerned about the above symptoms, discuss these in a routine appointment with your doctor. A blood test can check your thyroid function: if your free T4 is below the normal level and your TSH is high, this suggests hypothyroidism.

Some people have an enlarged thyroid gland, called a goiter. If very enlarged, this may be obvious to you, but your doctor will examine your neck for more subtle changes, and they may order a scan.

The most common cause of hypothyroidism is an autoimmune disease called autoimmune thyroiditis. This causes your body to make antibodies against your own thyroid gland, causing the gland to not make enough thyroxine, so that hypothyroidism gradually develops. This is more likely in those aged 30 to 50, in women, and if you or your close family members suffer other autoimmune conditions, such as type 1 diabetes, celiac disease, pernicious anemia or vitiligo.

You may have acquired hypothyroidism as a result of thyroid surgery or radiotherapy, in which case you are unable to produce enough thyroxine yourself.

Iodine deficiency is a common cause in some parts of the world and often causes a goiter. This is rare in the US or other industrialized countries.

What happens after diagnosis?

Your doctor is experienced at managing hypothyroidism and will likely start you on levothyroxine tablets to boost your thyroxine level. You will have regular blood tests to adjust the dose until you are symptom-free and thyroid function levels have returned to normal. It takes time for medications to work – weeks rather than days – and levels are typically re-checked no sooner than two months after any changes.

Once stabilized on treatment, your doctor will check your thyroid function every six to 12 months or sooner if symptoms return. If control is difficult or there are exceptional circumstances, your doctor may refer you for specialist management under an endocrinologist.

When should I seek urgent help?

It's extremely rare for hypothyroidism to result in a medical emergency, but if left untreated, there's a risk of myxedema. People around you will notice profound weakness, drowsiness, confusion, or psychosis, a slowing of the heart rate, and a drop in body temperature. You may lose consciousness and enter a myxedema coma, which is life-threatening. Any warning symptoms are reason to attend the emergency department for immediate treatment.

Are there any long-term effects?

You can avoid any long-term effects if you keep your thyroid levels well-controlled.

If left untreated or inadequately treated over a long period, you may suffer cardiovascular problems, including heart failure and poor blood flow to your kidneys and other organs. You may also risk damage to the nerve endings in your arms and legs, called peripheral neuropathy.

If trying to conceive, it can cause fertility problems which can be reversed with optimal treatment. Hypothyroidism in pregnancy is usually under specialist guidance, and levels are closely monitored to ensure you and your baby are kept in the best health. Levothyroxine is usually increased at the start of pregnancy.

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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