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
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 (Table 44.1) show low T4 and T3 levels
with a high TSH (except in the rare event of pituitary
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.
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