Hypothyroidism or underactive thyroid function, occurs when the thyroid gland fails to produce sufficient amounts of the thyroid hormones T4 and T3. There are four main causes:
Hypothyroidism can also be caused by disease of either the pituitary gland or the hypothalamus. This is because normal function of the thyroid gland depends on carefully regulated secretion of thyroid stimulating hormone (TSH) from the pituitary gland and thyrotropin releasing hormone (TRH) from the hypothalamus. Another important, but transient form of hypothyroidism occurs with postpartum thyroiditis or subacute thyroiditis.
Clinical Features
Hypothyroidism affects approximately 2 individuals in 100, its prevalence increasing with age. The signs and symptoms occur because there is a deficiency of thyroid hormone secretion and all metabolic processes "slow down." The patient has poor appetite, intolerance to cold, dry, coarse, skin, brittle hair, tiredness, a croaky, hoarse voice, constipation, and muscle weakness. Examination may reveal dry, scaly, cold, pale skin, a thickening of the skin and underlying tissues (called myxedema), very slow reflexes and a slow heart rate. The thyroid gland can be normal in size, increased (goiter) or non palpable. The patient can have poor memory retention. The diagnosis of hypothyroidism is confirmed by finding an increased level of TSH.
Neonatal Hypothyroidism
Newborn babies are tested using a "heelpad blood-spot test." Neonatal hypothyroidism is caused, in most babies by the absence or underdevelopment of the thyroid gland. In other cases, proteins necessary for the production of thyroid hormones are not properly functioning. Thyroid hormones are essential for brain development and growth. During the pregnancy, maternal thyroid hormones cross the placenta and provide for some of the foetal needs. New-born infants with hypothyroidism that are not treated develop cretinism characterised by severe body and mental defects. These include mental retardation, poor vision, thick dry skin, protrudent tongue, muscle weakness, severe lethargy and tiredness. If diagnosed and treated soon after birth, growth and mental development can proceed relatively normally.
Much of the research work in making an early diagnosis of Neonatal Hypothyroidism was carried out in Canada by Dr. J.H. Dussault at Laval University in Quebec City.
Subclinical Hypothyroidism
Borderline hypothyroidism is quite common, and can represent a challenging clinical diagnosis. The hallmark is that of an elevated TSH concentration, with normal thyroid hormone levels. There may be no symptoms, or very vague symptoms, associated with this condition.
It is important to make the correct diagnosis because once treatment is started it is usually lifelong. TSH is a very sensitive marker of even minor degrees of hypothyroidism.
Treatment
Treatment of hypothyroidism involves thyroid hormone replacement in the form of a small pill, daily, for life. This is now given in the form of thyroxine or T4 ("Eltroxin®" or "Synthroid®" or "Euthyrox®"), a synthetic hormone which has few impurities, very few side effects and produces almost no allergic reactions. The average replacement dose of dose of thyroxine in adults is 1.6 micrograms/kg of body weight. For the majority of patients, there is no proven additional benefit to add T3 ("Cytomel®"), since T4 is converted to T3, and the dosage is set to provide a normal T3 level. Once the dose has been established, it is usually stable. Periodic testing of TSH levels is used to assess that the treatment with thyroxine continues to be adequate. Major stress, pregnancy or illness can sometimes increase the need for thyroid hormone. Too much thyroxine causes symptoms of hyperthyroidism whereas symptoms of hypothyroidism persist with too little. The correct dose is determined by remeasuring the TSH levels taken 5 to 6 weeks after introducing or modifying the replacement dose.
Other Forms of Thyroid Hormone
There are many other forms of thyroid hormone but it is very unusual to prescribe any of these. Some preparations such as thyroid extract and crude thyroid preparations may contain variable amounts of thyroid hormones. They produce variable effects and an unpredictable response to treatment. Triiodothyronine (T3), which is much more potent than thyroxine is also given on occasion. This drug has a short life span in the blood and, therefore, its effect is not prolonged over 24 hours. If given, for selected patients, it is usually associated with T4 replacement.
Duration of Treatment
Assuming that the diagnosis of hypothyroidism was correct, treatment for thyroid hormone should always be continued for life. The cause of thyroid failure is likely to be progressive and permanent.
Hypothyroid patients should not stop taking thyroid hormone. Thyroid hormone treatment must be continued even when the patient develops other illnesses, although the dosage may have to be altered.
Treatment of Pituitary or Hypothalamic Hypothyroidism
The treatment of hypothyroidism caused by failure of the pituitary or the hypothalamus is also thyroxine. Pituitary or hypothalamic failure are both very rare compared to failure of the thyroid gland. In these cases, other hormone deficiencies may exist which must be identified and treated as well. In theses cases T4 levels are used to monitor the adequacy of the replacement therapy.