Understanding Thyroid Disorders
Introduction
What is the thyroid gland?
The thyroid gland lies in the front of the neck between the skin and the voice box. It has a right and left lobe each about five centimetres in length and joined in the midline. The entire gland weighs less than an ounce (about 20 grams). Despite its small size it is an extremely important organ which controls our metabolism and is responsible for the normal working of every cell in the body.
Thyroid hormones
The thyroid gland achieves this control by manufacturing the hormones (see Glossary) thyroxine (T4) and triiodothyronine (T3) and secreting them into the bloodstream.
Iodine is an important constituent of these hormones. There are, and three atoms of iodine in each molecule of triiodothyronine or T3. four atoms of iodine in each molecule of thyroxine, hence the abbreviation T4
Doctors believe that T4 starts to be active only when it is converted, mainly in the liver, to T3 by the removal of one atom of iodine. In parts of the world where there is a severe lack of iodine in the diet, such as the Himalayas, there is not enough iodine for the thyroid gland to make adequate amounts of T3 and T4. In an attempt to compensate, the thyroid gland enlarges to form what is known as a goitre, which is visible. If this extra manufacturing capacity is still inadequate, the patient develops an underactive thyroid gland.
Iodine deficiency is not present in the UK. Sometimes too much iodine in the diet causes the thyroid gland to produce excessive amounts of thyroid hormones. This can also be a result of medication.
Balancing the hormones
In healthy people the amounts of T3 and T4 in the blood are maintained within narrow limits by a hormone known as thyroid-stimulating hormone (TSH) or thyrotrophin. TSH is secreted by the anterior pituitary gland which is a pea-sized structure, hanging from the undersurface of the brain just behind the eyes, and enclosed in a bony depression in the base of the skull.
When thyroid disease causes thyroid hormone levels in the blood to fall, TSH secretion from the pituitary is increased; when thyroid hormone levels rise, TSH secretion is switched off – a relationship known as ‘negative feedback’, familiar to engineers and biologists.
Hypothyroidism and hyperthyroidism
If your GP suspects that you may have an underactive thyroid gland (hypothyroidism), his or her diagnosis can be confirmed by sending a sample of your blood to the laboratory for analysis. Low levels of T3 and T4 and high levels of TSH in your blood mean that your doctor was right. Similarly, the diagnosis of an overactive thyroid gland (hyperthyroidism) is confirmed by high levels of T3 and T4 and low levels of TSH. The results will be available within a few days.
Patients with uncomplicated hypothyroidism will not usually be referred to hospital and your GP can prescribe and monitor your treatment. Most patients with hyperthyroidism or with abnormal growth of the thyroid gland will be referred to a hospital specialist for further investigation and advice about treatment.
Thyroid disease is common and hyperthyroidism, hypothyroidism or abnormal growth or enlargement of the gland (goitre or thyroid nodule) affects about one in 20 people. Most diseases of the thyroid can be successfully treated, and even thyroid cancer, which is rare, may not lead to a reduction in life expectancy if detected early and treated appropriately.
Thyroid disease often runs in families but in an unpredictable manner, and certain forms are associated with an increased risk of developing conditions such as diabetes mellitus or pernicious anaemia. All types of thyroid disease are more common in women.
The following chapters deal with each of the most common thyroid disorders individually.
Case history
Ahmed was born in a village in the high mountains of northern Pakistan where he spent most of his childhood. At the age of 20 he came to London to study engineering when, at a routine medical examination, he was noticed to have a goitre. He felt well and all the thyroid tests were normal.
The cause of the goitre was attributed to iodine deficiency when Ahmed told the doctor that most of the people in his village also had a goitre. His diet had contained enough iodine to prevent the development of hypothyroidism, but his goitre is likely to remain, even though he has decided to live the rest of his life in a part of the world where there is an adequate amount of iodine in his diet.
KEY POINTS
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Thyroid disease is common, affecting around one in 20 people
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More women than men are affected
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Your GP can diagnose the condition with a simple blood test
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Treatment is usually successful, and even thyroid cancer can be cured if caught early
