Hypothyroidism (underactive thyroid gland, myxoedema) is a condition in which the thyroid gland produces too little thyroid hormone (thyroxine) which results in effects on the whole body.

The thyroid gland, which is in the front of the neck, controls the rate of at which the body's cells work (the metabolic rate). Many people with a mildly underactive thyroid gland will not notice any symptoms. In hypothyroidism, the rate of metabolism is decreased, and this results in most of the symptoms:

• feeling sluggish and tired
• tendency to feel cold
• weight gain
• depression
• dry skin and hair
• constipation
• heavier periods

Hypothyroidism is more common than thyrotoxicosis. In most cases it is because the thyroid cells begin to fail, and thus less thyroid hormone (thyroxine) is produced. This may occur without obvious cause, but may follow:

• the body developing an immune reaction against itself (an auto-immune reaction) which destroys the thyroxine producing cells
• an operation to remove part of the thyroid gland (usually for thyrotoxicosis), often years earlier
• radioactive iodine treatment for thyrotoxicosis (¹³¹I)
• thyrotoxicosis in earlier life
• treatment with drugs you may be taking, such as lithium or amiodarone

Very rarely hypothyroidism may follow failure of the production of the hormone (thyroid stimulating hormone, TSH) that is produced by a gland hanging from the underside of the brain (pituitary gland) to drive the thyroid gland. This is known as secondary hypothyroidism.

In the presence of symptoms such as some or all of those above, the doctor is likely to arrange for a blood test to check on the amount of thyroid hormone (thyroxine) and other tests, looking at other hormones which control or are produced by the thyroid gland. These tests are collectively known as thyroid function tests. You may also have tests done to look for antibodies to thyroid (auto-antibodies).

In hypothyroidism the level of TSH (thyroid stimulating hormone) is usually elevated, and the level of thyroxine is low.

Your doctor is likely to prescribe thyroxine (also known as levothyroxine or T4). In some cases you may be prescribed an alternative thyroid hormone, triiodothyronine (T3 or liothyronine).

If you are elderly or have any history suggestive of heart problems (eg angina or heart failure), the doctor will start you on a lower dose, as thyroxine treatment increases the rate of work of all the cells in the body. Your system has probably been used to functioning at a lower rate for some time, and the increase in energy will cause extra work for the heart. It would be harmful to continue without treating an underactive thyroid gland, but, as with starting an exercise programme, it is sensible to build up to a higher level of metabolic rate in stages.

The way in which treatment with thyroxine is monitored is by visits to the doctor, who might ask questions about how you feel, check your weight and pulse, and sometimes check your reflexes (which relax slowly in hypothyroidism).

The main part of follow up, however, is regular checking of the level of TSH (thyroid stimulating hormone) in your blood. If the TSH level is high this means that you are not having adequate thyroxine to allow for your underactive thyroid gland. In this case the pituitary gland works harder, producing more TSH, in an unsuccessful attempt to make the thyroid produce more thyroxine. If the TSH level is normal then you are having adequate thyroxine to replace the lacking hormone from your thyroid gland (replacement therapy).

Initially you will have TSH blood tests every few weeks or months. If the TSH is high your doctor is likely to increase the dose of thyroxine. Most people do not need a dose of more than 150 micrograms of thyroxine per day.

• British Thyroid Foundation

Books

Thyroid Disease: The facts, by R I S Bayliss, W M G Tunbridge (Oxford University Press)

Aimed at patients with thyroid disease and their relatives, this book offers an authoritative account of the symptoms, causes and treatments of the various ailments of the gland.

Living well with Hypothyroidism, by Mary J Shomon

Patient-written, patient-oriented book.

Understanding Thyroid Disorders (BMA Family Doctor Books), by Dr Anthony Toft

Reference source on thyroid disorder, providing case histories, key points, questions, answers, and practical tips for overcoming problems.