The thyroid gland was once described as the conductor of the body's orchestra. Situated in the neck like a bow tie, just above the breastbone, the thyroid secretes hormones that act on almost every cell in the body to control their rate of metabolism.
The gland literally has the ability to speed us up or slow us down.
Problems with the thyroid particularly affect women and are most commonly diagnosed in their 40s. This is especially true of an underactive thyroid.
While an underactive gland will sometimes cause acute symptoms, it is usually something that comes on slowly and insidiously, as the following case illustrates.
Sarah is 42 and a mother of four. She has always struggled with her weight, but about two years ago she gained a moderate amount that just would not shift, despite dieting and increasing amounts of exercise.
At the same time, she felt cold a lot of the time and recalls sitting on a beach in Mayo with her coat on last summer, when all around her were comfortable in T-shirts and shorts.
Pains in her hands and feet were the next symptom, which progressively worsened. And her hairdresser remarked on changes to her hair; Sarah herself noticed her normally greasy hair was now very dry.
But it was not until she became breathless while walking around on a pre-Christmas shopping trip that her husband suggested she see a doctor. He was worried that his wife was anaemic, but when the results of a broad range of blood tests came back, they confirmed low levels of thyroid hormone in Sarah's blood.
Often, patients with underactive thyroids will have quite vague symptoms. Tiredness, mild depression, slight joint or muscle pains and a small amount of weight gain do not point to any particular condition.
Many of us might put them down to the effects of advancing age, and doctors themselves may overlook the possibility of hypothyroidism.
There is an argument for performing thyroid-function tests as a general screening exercise in the middle-aged and the elderly. In addition, younger women who have period or infertility problems should have their thyroid function checked.
The interpretation of thyroid-function tests is not always straightforward. Two principal measurements are taken from a single blood test: the level of T4, or thyroid hormone, and the level of TSH, or thyroid-stimulating hormone.
TSH is produced by the pituitary gland, in the brain, acting as a messenger to stimulate the thyroid gland to release more thyroid hormone.
A high TSH and a low T4 level confirms hypothyroidism, reflecting the feedback between the thyroid and the pituitary. As the thyroid fails to produce adequate levels of T4, TSH levels climb in an effort to kick-start production.
It is not uncommon to get a result that shows slightly elevated TSH but normal T4 levels in the blood. The patient may have some mild symptoms or may be completely well. There is much debate about what to do in these circumstances.
Does the doctor continue to monitor the situation; say, by repeating the test in three to six months' time?
Or should he recognise the likelihood that the result reflects a "failing" thyroid gland and begin replacement therapy immediately?
L-thyroxine is the name for manufactured thyroid hormone. The exact dose is determined by ongoing blood tests - once TSH and T4 levels normalise, the correct dosage is being used.
Most patients will start on between 50 and 100 micrograms, taken as a single daily dose; the final amount will be determined by the results of a blood test taken every six weeks until the blood levels settle. An annual blood test will then suffice in order to monitor therapy.
The advantage of initiating therapy in patients not yet showing symptoms of thyroid disease is that doctors can prevent them developing the condition; also, there is some evidence to suggest early replacement may help to reduce the risk of subsequent coronary heart disease.
L-thyroxine is a relatively benign drug, with caution required only in the elderly. Over-replacement can cause disturbance of heart rhythm and, in patients with angina, it could precipitate a heart attack.
Borderline hypothyroidism usually reflects an initial attack on the thyroid by the body's immune system.
Such autoimmune disease is also called Hashimoto's thyroiditis, after the Japanese surgeon who first described it, in 1912.
The thyroid can be enlarged also. The gland thickens as the antibodies produced by the immune system cause changes in its cellular structure.
A thyroid swelling or enlargement is referred to as a goitre. It is possible to have an enlarged gland that functions normally.
Hypothyroidism is a relatively easy condition to treat. Lifelong medication is the norm and, as Sarah found to her surprise, symptoms tend to resolve quickly after treatment begins.
E-mail Dr Muiris Houston, Medical Correspondent, at mhouston@irish-times.ie or leave a message at 01-6707711 ext 8511. He regrets he cannot reply to individual medical problems
