What is an underactive thyroid?

An underactive thyroid means that the thyroid gland is not producing enough thyroid hormone to perform its necessary metabolic functions. If we do not produce enough thyroid hormone then we are said to be ‘hypothyroid’. You can measure your thyroid hormones with a blood test.

Symptoms of hypothyroidism

The lack of 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 hypothyroidism

There can be a number of 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 against the thyroid gland, which result in damage to the gland.

The gland stops functioning normally, and does not produce enough thyroxine, leading to hypothyroidism.

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

Autoimmune thyroiditis with a goitre (an abnormal swelling of the neck) is called Hashimoto’s disease. The other form of the condition is without a goitre, and is called Atrophic autoimmune hypothyroidism.

Surgery or treatment to the thyroid gland

An underactive thyroid gland can occur as a side effect of treatment to a previously overactive thyroid gland.

Postpartum thyroiditis

This is a, usually, transient abnormality in thyroid function following pregnancy. It can result in increased or reduced thyroid function and sometimes even one after another.

If there is also a presence of thyroid antibodies, along with postpartum thyroiditis, this increases the risk of progression to permanent hypothyroidism.

Iodine deficiency

The body needs iodine to make thyroxine, so a lack of iodine in the diet can cause hypothyroidism.

Iodine deficiency is the most common cause of hypothyroidism worldwide, however is not common in the UK.


Some medications used to treat other illnesses can cause hypothyroidism.

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


Blood results in hypothyroidism

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.

Therefore, when thyroid hormone levels (T3 and T4) are incorrectly lowered, as we would see in hypothyroidism, we can expect TSH levels to be high.

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

Subclinical hypothyroidism

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

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

It is important to note that 20-50% of people with SCH will go on to develop ‘clinical’ hypothyroidism i.e. raised TSH and low free T4. The risk of this happening is increased further if a person also has antibodies to TPO.

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.

It is thus important for your doctor to rule out other causes of your symptoms before ascribing them to subclinical hypothyroidism.


TSH – Thyroid Stimulating Hormone

T3 – Triidothyronine

T4 – Thyroxine

TPO – Anti-thyroid Peroxidase

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Claire is the Inbound and Content Manager here at Thriva. Her mission is break through all the clutter of information that is out there relating to health and bring you informative, easy to digest, actionable insights.