Hypothyroidism

A scheme for classifying hypothyroidism is given in Table 44.2.

Cretinism  (foetal or infantile hypothyroidism)

 

Sporadic c is due to complete or near complete failure of thyroid development (partial failure causes juvenile myxoedema): the parents and other children may be perfectly normal. In endemic areas, goitrous cretinism is common, and is due to maternal and foetal iodine deficiency. Immediate diagnosis and treatment with thyroxine within a few days of birth are essential if physical and mental development are to be normal, or if further deterioration is to he prevented when damage has already occurred in utero. Hypothyroidism occurs in I in 4000 live births and for this reason, in the UK, there is routine biochemical screening of neonates for hypothyroidism using TSH assay on a simple heel-prick blood sample. Women under treatment with antithyroid drugs may give birth to a hypothyroid infant.

Adult hypothyroidism

The term myxoedema should be reserved for severe thyroid failure and not applied to the much commoner mild thyroid deficiency.

 The signs of thyroid deficiency are:

 

  bradycardia;

  cold extremities;

  dry skin and hair;

periorbital puffiness;

  hoarse voice;

  bradykinesis slow movements;

  delayed relaxation phase of ankle jerks.

 

The symptoms are:

 

  tiredness;

  mental lethargy;

  cold intolerance;

  weight gain;

  constipation;

  menstrual disturbance;

  carpal tunnel syndrome.

Comparison of the facial appearance with a previous photo­graph may be helpful.

  Thyroid function tests

Thyroid function tests (Table 44.1) show low T4 and T3 levels with a high TSH (except in the rare event of pituitary failure). High serum titres of antithyroid antibodies are characteristic of autoimmune disease.

Treatment

Oral thyroxine (0.10—0.20 mg) as a single daily dose (because of its prolonged action) is curative. Caution is required in the elderly or those with cardiac disease and the replacement dose is then commenced at 0.05 mg daily and cautiously increased. If a rapid response is required, tri-iodothyronine (20 mg three times a day) may be used. The signs and symptoms of hypothyroidism are accentuated. The facial appearance (Fig. 44.11) is typical, and there is often supraclavicular puffiness, a malar flush and a yellow tinge to the skin. Myxoedema coma occurs in neglected cases and carries a high mortality; the body temperature is low and the patient must be warmed slowly: 1 g of intravenous hydrocortisone (in divided doses) should be given daily, and intravenous tri-iodothyronine in slowly increasing doses.

Autoimmune thyroiditis

The so-called primary or atrophic myxoedema is now considered to be an autoimmune disease similar to chronic lymphocytic (Hashimoto’s) thyroiditis (see below) but ‘without goitre formation from TSH stimulation. Because of the delay in diagnosis the hypothyroidism is usually much more severe than in goitrous autoimmune thyroiditis.

Dyshormonogenesis and goitrogens

Genetically determined deficiencies in the enzymes controlling the synthesis of thyroid hormones, if severe, are responsible for goitre formation with hypothyroidism. If of moderate degree, a simple (euthyroid) goitre results. Similarly goitrogens may produce a goitre with, or without, hypothyroidism.

A number of nonendemic goitrous cretins has been born to a group of itinerant tinkers living in Scotland who intermarry (Hutchison, Scotland). This was due to a deficiency of the enzyme dehalogenase. When thyroglobulin is broken down, uncoupled iodotyrosines are liberated as well as T3 and T4. They are broken down by the enzyme dehalogenase and the iodine retained within the thyroid. If dehalogenase is deficient, iodotyrosines pass into the blood, and are excreted in the urine and this may result in iodine deficiency and goitre formation. Another classic example of dyshormonogenesis is Pendred’s syndrome, where goitre is associated with congenital deafness. This is due to a deficiency of peroxidase, the enzyme responsible for organification of trapped iodine. Defects in thyroglobulin synthesis are also recognised in dyshormonogenesis.