The thyroid is an important endocrine gland located in the neck. Two lobes on either side are connected across the midline by a narrow segment of tissue called the isthmus. The function of the thyroid is to manufacture, store and secrete two thyroid hormones, named thyroxine and tri-iodothyronine.
Iodine is an essential ingredient for this production, and without it, problems rapidly occur and increase. Each day the body needs between 100 and 200 mcg of iodine. This usually comes from marine fish, and from vegetables grown in soil containing iodine. When this is digested, it is trapped by the thyroid gland, under stimulation from the thyroid-stimulating hormone (TSH) of the pituitary gland.
Two important medicaments called thiouracil and carbimazole have the opposite effect, preventing it from being trapped and utilised by the thyroid gland. This is important, for they are often used in treating overactive thyroid glands. Under the influence of TSH, the two thyroid hormones, thyroxine and tri-iodothyronine, are released into the bloodstream.
Production of these hormones is largely influenced by a naturally occurring “feedback” system. The higher the level of the hormones in the blood, the less TSH is produced, and vice versa. In this way, relatively stable levels are maintained. It is a very ingenious and workable system.
The functions of the thyroid hormones in the body are very widespread. They affect the rate at which all body cells work. All normal development is dependent on them. Muscle growth, sexual development, the rate at which calories are burnt up, the rate and output of the heart are all related. The breakdown and utilisation of protein is greatly influenced by it. This is clearly shown by the weakness that develops in a patient with an overactive thyroid (hyperthyroidism) and the symptoms that occur.
Many tests have been devised to measure the activity and presence of the thyroid hormones. These tests are aimed at helping to diagnose accurately the various disorders that can occur with this gland. Anyone who has been investigated for thyroid problems will be well acquainted with names such as BMR (basal metabolic rate), once popular but now rarely, if ever used, protein-bound iodine test, tri-iodothyronine (T3) resin uptake test, serum total thyroxine and thyroxine (T4) resin uptake test, free thyroxine index, radioactive iodine uptake, TSH stimulation test, tri-iodothyronine suppression test and thyroid scan.
In recent years it has been found that thyroid antibodies occur, and a search may be made for these. These are the so called thyroidal antibodies. More recently an antibody referred to as LATS (short for long-acting thyroid stimulator) has been found in some patients with Graves’ disease.
The general principles of thyroid disorders will be discussed. First there is the simple nontoxic goitre (enlargement of the thyroid) occurring just because the diet is deficient in iodine. It represents the body’s efforts to make amends for this.
The gland may be overactive or underactive. Overactivity gives rise to the condition known as hyperthyroidism, commonly called Graves’ disease. A similar condition tending to occur in older persons is called nodular toxic goitre. If the thyroid gland is underactive, the resultant condition is referred to as hypothyroidism. In infants this is called cretinism, and in older patients it is called tnyxoedema.
Cancer of the thyroid is relatively rare, but is important, and it will be discussed briefly. Also, mention will be made of another condition called thyroiditis.