The thyroid's job is to take iodine out of the food we eat, and then produce iodine-containing hormones (thyroxine and tri-iodothyronine) that regulate the speed at which we use the energy from the food.

Less energy, for instance, would be needed when we're sleeping and more when we are exercising or active.

Iodine also stimulates cell growth and helps the body regulate its energy levels.

Underactive Thyroid

The thyroid also supports many of our bodies' functions, from controlling our temperature, pulse rate, cholesterol level, muscle strength and digestive function, to dictating our mood.

The most common disorder of the thyroid is hypothyroidism -- an underactive thyroid. In this situation, the thyroid doesn't derive enough iodine, cannot produce enough hormone, and therefore shrivels away.

Over time, the symptoms would be weight increase, a slowing of the heart rate, thinning hair, difficulty in swallowing and general fatigue.

Although hard to detect -- the symptoms are often confused with general aging or even stress -- it is easily diagnosed by a simple blood test and can be treated with synthetic hormone tablets.

Graves' Disease

If the gland becomes overactive, it produces too much thyroxine (hyperthyroidism). Symptoms include weight loss despite an increase in appetite, increased heart rate, shaking hands, protuberant eyes and a swollen thyroid gland known as a goiter.

This is a condition known as Graves' disease and is more common among women in their 30s and 40s. Treatment is complex for this rarer condition and can include surgery and drug therapy.

It's important to look after your thyroid by ensuring you eat enough iodine -- one ten thousandth of a gram per day.

Fortunately, a balanced diet, including fresh fruit and vegetables, should give you your daily allowance. Any excess iodine is lost in urine.