Understanding thyroid disorders: underactive and overactive

What the thyroid does

The thyroid gland sits at the front of the neck and produces hormones, mainly thyroxine, that set the pace of your metabolism — how fast your body burns energy. These hormones affect almost everything: your heart rate, temperature, weight, digestion, mood and energy levels. The gland is controlled by the pituitary, a small gland in the brain, which releases a hormone called TSH to tell the thyroid how much to produce. This forms a feedback loop that normally keeps hormone levels steady. When this balance is disturbed, and the thyroid makes too little or too much hormone, the effects are felt throughout the body, which is why thyroid symptoms can be so wide-ranging and varied.

Underactive thyroid (hypothyroidism)

An underactive thyroid means the gland does not make enough hormone, so the body slows down. Symptoms tend to develop gradually and can include tiredness, weight gain, feeling cold, dry skin, constipation, low mood and slowed thinking. Because these are common and non-specific, the condition is easily missed or mistaken for other things. The most common cause in the UK is an autoimmune condition, where the immune system gradually damages the thyroid. It can also follow thyroid surgery or certain treatments. An underactive thyroid becomes more likely with age and is far more common in women. It is diagnosed with a simple blood test and is usually very treatable.

Overactive thyroid (hyperthyroidism)

An overactive thyroid means the gland makes too much hormone, so the body speeds up. Symptoms can include a fast or irregular heartbeat, weight loss despite a good appetite, feeling hot and sweaty, anxiety or irritability, shaky hands, and difficulty sleeping. Some people notice a swelling in the neck (a goitre), and in one common cause, called Graves’ disease, the eyes can become affected. Graves’ disease is an autoimmune condition and a frequent cause. An overactive thyroid needs proper treatment, as if left unchecked it can strain the heart and, rarely, cause a dangerous flare-up. Like an underactive thyroid, it is diagnosed with blood tests.

How thyroid problems are diagnosed

Thyroid function is checked with a simple blood test, usually starting with TSH, the hormone from the pituitary that regulates the thyroid. A high TSH generally suggests an underactive thyroid, while a low TSH suggests an overactive one; the thyroid hormones themselves are often measured too. Sometimes tests for thyroid antibodies help confirm an autoimmune cause. If the gland is enlarged or a lump is felt, a scan or other tests may be arranged. Because symptoms are so varied and common, a blood test is the reliable way to tell whether the thyroid is the cause. If your GP suspects a thyroid problem, this test is usually the first step.

Treatment and living well

An underactive thyroid is treated by replacing the missing hormone with a daily tablet, which restores normal levels; most people feel much better once the dose is right, and treatment is usually lifelong with regular blood tests to check the balance. An overactive thyroid has several treatment options, including medicines to reduce hormone production, a form of radioactive iodine treatment, or occasionally surgery; the choice depends on the cause and your situation, guided by a specialist. Both conditions are very manageable. Take medicines as prescribed, attend your blood-test reviews, and tell your GP if symptoms return or if you become pregnant, as thyroid treatment may need adjusting.