Hypothyroidism

Hypothyroidism means the thyroid gland is underactive and does not produce enough thyroid hormone. The thyroid is a small, butterfly-shaped gland in the front of the neck, and its hormones help control how quickly the body uses energy.

When hormone levels are low, many body processes slow down. This is why an underactive thyroid can affect energy, weight, mood, temperature and many other functions across the whole body.

Hypothyroidism is common and very manageable. Once diagnosed, it is usually treated effectively with a daily tablet that replaces the hormone the thyroid is no longer making in sufficient amounts.

Symptoms tend to develop slowly over months or years and can be subtle at first. Common ones include tiredness, weight gain, feeling cold, dry skin, constipation, low mood, slowed thinking, muscle aches, and thinning hair.

In women, periods may become heavier or irregular. Because these symptoms are general and develop gradually, hypothyroidism is sometimes mistaken for stress or normal ageing, and a blood test is needed to confirm it.

The most common cause is an autoimmune condition in which the immune system mistakenly attacks the thyroid gland, gradually reducing how much hormone it can make. This is sometimes called Hashimoto's disease.

Other causes include previous thyroid surgery, treatment of an overactive thyroid, radiotherapy to the neck, certain medicines, and a lack of iodine in the diet in some parts of the world. Hypothyroidism can also be present from birth, which is why newborns are screened for it in many countries.

Hypothyroidism is diagnosed with a blood test that measures thyroid hormone levels. The main marker is thyroid-stimulating hormone (TSH), which tends to rise when the thyroid is underactive, often measured together with a thyroid hormone called T4.

If the tests confirm an underactive thyroid, a doctor may also check for the antibodies linked to the autoimmune cause. Blood tests are then repeated periodically to guide treatment and to make sure the dose of medication is right.

The standard treatment is a daily tablet called levothyroxine, which is a man-made version of the thyroid hormone the body is missing. It simply replaces what the thyroid is not making, restoring normal hormone levels.

Levothyroxine is usually taken once a day, often on an empty stomach. The dose is adjusted using blood tests until levels are stable, and most people then stay on a steady dose. For most people with permanent hypothyroidism, treatment is lifelong.

It is important to take the medication regularly and not to stop without medical advice, even once you feel well, because symptoms return if hormone levels fall again. Routine blood tests help keep the dose correct over time.

Most cases cannot be prevented because they are caused by autoimmune disease or medical treatment, although having enough iodine in the diet is important where deficiency is common. With treatment, the outlook is very good and most people feel well and live normally.

If a significantly underactive thyroid is left untreated for a long time, symptoms can worsen and it may contribute to problems such as raised cholesterol and heart strain. Very rarely, severe untreated hypothyroidism can become a medical emergency. Treatment with levothyroxine prevents these problems in the great majority of people.