The thyroid gland produces two hormones triiodothyronine (T3) and thyroxine (T4) which is converted in cells to T3. Both are made up of atoms of the trace mineral iodine. If your thyroid gland doesn't produce enough of these hormones, everything in your body slows right down; you have less energy and feel tired and fatigued, develop constipation and start to gain weight.
Other thyroid conditions include hyperthyroidism, where the thyroid gland is overactive and produces too much thyroxine, but hypothyroidism is the more common of the two.
The most common cause of hypothyroidism is an autoimmune thyroiditis, (also known as Hashimoto's thyroiditis), where your body makes antibodies which attack your thyroid gland. Scientists still don't know what triggers the body to do this – theories include a bacterial or viral cause, or a genetic defect.
Other causes include treatment for hyperthyroidism which can sometimes reduce thyroxine levels too much and thyroid cancer.
Less common causes of hypothyroidism include hormone changes triggered by pregnancy, radiation treatment for head and neck cancers, thyroid surgery and the side effects of some prescription drugs.
The trace mineral iodine is essential for the production of thyroid hormones. Although most people get enough iodine from their diets, there have recently been concerns some adolescent girls and pregnant women might not get adequate amounts. A UK study published in 2011 found two-thirds of 700 girls sampled had iodine levels below the acceptable minimum (100ug/L) set by the World Health Organisation.
Symptoms usually develop slowly, over many years, and some such as feeling tired, constipation and weight gain can be wrongly attributed to just getting older.
The most common symptoms include tiredness, lack of energy, poor concentration, depression and feeling cold. Other symptoms include: dry skin; a hoarse voice; a puffed-up face; muscle weakness and pain; thinning hair; painful joints; a slowed heart rate; and heavier menstrual periods.
If hypothyroidism goes untreated, you may also develop memory problems and a swelling of your thyroid gland called a goitre.
A thyroid function blood test can diagnose an underactive thyroid and can be ordered by your GP. This test measures the level of thyroid-stimulating hormone (TSH), responsible for stimulating the gland to produce thyroxine. If a raised TSH level is found, you're likely to have an underactive thyroid as when thyroxine levels are low, your thyroid produces more TSH. Levels of thyroxine are also measured.
A TSH test may also pick up a condition called subclinical hypothyroidism (sometimes known as mild thyroid failure). This is where people have raised levels of TSH but normal levels of thyroxine. Some patients with subclinical hypothyroidism will have symptoms of hypothyroidism and others will not, but with each year that passes there is a two to five per cent risk these patients will develop hypothyroidism.
Who gets hypothyroidism?
Hypothyroidism affects eight per cent of all women, and 10 per cent of women aged over 55. Around three per cent of men are said to have sub-clinical hypothyroidism. You're at increased risk of hypothyroidism if you are a woman, are over 60, have an autoimmune disease or a family history of thyroid disease.
Babies are sometimes born with an underactive thyroid and all are screened at birth for this.
An underactive thyroid can be treated with daily Levothyroxine tablets, an artificial form of thyroxine – and once diagnosed you will have to take these for the rest of your life.
Guidelines suggest your doctor prescribes an initial dose of 50 to 100mcg once daily, taken before breakfast or caffeinated drinks. This should then be gradually increased by 25 to 50mcg every three weeks according to how you respond. Once a stable dose is reached you should have follow-up thyroid function tests every four to six weeks.
The UK's National Institute of Health and Care Excellence (NICE) recommend people aged under 70 with a TSH level greater than 10mIU/L should be treated with Levothyroxine even if their thyroxine levels are in the normal range. It recommends a 'watch and wait' approach in those aged over 70.