Family doctor

Overview

• Hypothyroidism is a condition in which the amount of thyroid hormone in the body is below normal.
• It affects approximately 2 persons in 100, and is more common in women and with increasing age.
• Thyroid hormones are produced by the thyroid gland located in the lower part of the neck. They are chemical messengers that help regulate growth and the rate of chemical reactions (metabolism) in the body.
• Treatment is simple and effective but is usually lifelong.

Causes

Most cases of hypothyroidism are due to inadequate production of thyroid hormone by the thyroid gland, referred to as primary hypothyroidism.

The main causes of primary hypothyroidism are:

Inflammation of the thyroid gland (thyroiditis)

The most common type of thyroiditis is called Hashimoto's Thyroiditis. Antibodies are produced by the body which interrupt the normal function of the thyroid gland. It can run in families. Inflammation can also occur after pregnancy (postpartum thyroiditis), as a result of bacterial infection or after a viral illness.

A Birth Defect (Congenital)

One in 4000 babies are born with a non-functioning or absent thyroid gland resulting in hypothyroidism. It is very important to diagnose and treat this to prevent severe mental and growth retardation. All babies in New Zealand are tested at birth with a heel prick blood test.

Acquired

Hypothyroidism may occur as a result of surgical removal of the thyroid gland for treatment of thyroid cancer or an overactive thyroid gland. It may also occur as a result of some drugs such as radioactive iodine (used to treat an overactive thyroid gland), lithium (used to stabilize mood) and amiodarone (used to treat palpitations).

Severe Iodine deficiency

Iodine is needed to make thyroid hormone. In some countries (such as India and South America) the soil content of iodine is poor and hypothyroidism is more common. It is rare in the Western World since the addition of iodine to table salt.

Thyroid gland function is controlled by the pituitary gland, a small gland at the base of the brain. It produces a hormone called Thyroid Stimulating Hormone (TSH) if the body's thyroid hormone levels are low. This TSH stimulates the thyroid gland to make more thyroid hormone. In rare cases hypothyroidism is a result of reduced TSH production by the pituitary gland, referred to as secondary hypothyroidism.

What are the symptoms?

Patients with mild hypothyroidism may have no symptoms. This is referred to as "subclinical hypothyroidism". Early symptoms may be nonspecific, and with a very slow onset, may be wrongly attributed to aging. Symptoms become evident with more severe hypothyroidism and are related to the slowed metabolism of the body.

Symptoms may include some of the following:

• Fatigue
• Weakness
• Intolerance to cold
• Constipation
• Dry coarse skin
• Poor appetite
• Poor memory
• Croaky hoarse voice
• Weight gain
• Muscle cramps

Diagnosis

Your family doctor may suspect hypothyroidism on the above symptoms. A thorough clinical examination may also reveal an enlarged thyroid gland (goitre), thickening of the skin, slow reflexes and a slow heart rate.

The diagnosis is confirmed by blood tests. Levels of thyroid hormone are low in patients with hypothyroidism but may be normal in early or mild cases and only become low as the disease progresses. TSH levels are always elevated in primary hypothyroidism. In secondary hypothyroidism thyroid hormone levels and TSH levels are low.

Thyroid antibodies may also be found in the blood and are common in Hashimoto's Thyroiditis. Other blood tests may reveal an elevated cholesterol and an anaemia associated with hypothyroidism. The disease may coexist with diabetes and pernicious anemia (low vitamin B12).

What is the treatment?

Treatment of hypothyroidism is simple and effective. Thyroid hormone replacement is taken in the form of a small pill (thyroxine or eltroxin). Doses usually range from 0.05mg-0.2mg daily. Gradually increasing doses are given until the thyroid hormone level and TSH levels are normal again. The levels are monitored by regular blood tests. Once the dose has been established most patients stay on the same dose for life, with checkups and blood tests by their doctor once or twice yearly.

Thyroxine tablets should be taken on awakening at least 30 minutes before eating. Some bran or fibre foods may reduce absorption. Iron tablets and antacids should not be taken at the same time as they will also reduce absorption.

Side effects are very rare and are usually a result of not enough or too many tablets. Too much thyroid replacement can result in symptoms or an overactive thyroid gland (heat intolerance, anxiety, sweatiness, palpitations). Not enough thyroid replacement can result in the symptoms of hypothyroidism persisting.

When the patient is elderly or has heart disease it is important to start with a low dose and increase the dose very gradually.

Most patients with hypothyroidism lead absolutely normal lives with effective treatment and suffer no long-term problems.