Acquired Hypothyroidism

The most common cause of acquired hypothyroidism in iodine – sufficient regions is autoimmune thyroiditis. In iodine deficient counties, iodine deficiency remains the most common thyroid disease.

Other causes of childhood onset of hypothyroidism are:
1) Ectopic thyroid or dysgenesis – Late onset
2) Drug induced hypothyroidism e.g. amiodarone
3) Environmental goitrogens
4) Central hypothyroidism (Hypothalamic or Pituitary)
5) Radioiodine ablation or surgery.

Acquired hypothyroidism can occur at any age. These children present with growth failure, myxedematous change of skin, constipation, cold intolerance, decreased energy and an increased need for sleep. Scholastic performance is usually not affected. Bone age is delayed and is an indication of the duration of hypothyroidism. Adolescents have delayed puberty whereas younger children may have pseudoprecocious puberty. (Postulated to be due to high TSH concentrations binding to FSH receptor and subsequent stimulation).

Some children may present with headache and visual problems, which may be in long standing hypothyroidism due to hyperplastic enlargement of pituitary gland as a result of thyrotroph hyperplasia.

Treatment consists of thyroid hormone supplementation but in children with long standing hypothyroidism, catch up growth may be incomplete. The thyroxine initially should be replaced in low doses followed by gradual increase in the dose till euthyroid state is achieved and keeping the child on maintenance dose to decrease incidence of pseudotumor cerebri and cardiac failure. During the first year deterioration of school work, poor sleeping habits, restlessness and behavioral problems may occur but it is transient and partially ameliorated by starting at sub replacement supplementation and increasing slowly.

Autoimmune thyroiditis: Autoimmune thyroiditis is the most common cause of thyroid disease in children and adolescents leading to goiter or acquired hypothyroidism with/without a goiter.

A variety of different thyroid antigen antibodies are involved that block the thyrotropin receptors or inhibit enzyme activity leading to lymphocytic infiltration of the thyroid and hyperplasia. Later on follicles may atrophy. Certain haplotypes such as HLA-DR4, HLA-DR5 are associated with increased risk of goiter and thyroiditis and HLA-DR3 with atrophic variety of thyroiditis.

Thyroid antiperoxidase antibodies (TPO Abs/ antimicrosomal antibodies), antithyroglobulin antibodies and thyrotropin receptor blocking antibodies may be positive in autoimmune thyroiditis.

The disease in children is more common in females than in males with onset usually after 4 years of age with a peak during adolescence. Children with Down’s syndrome and girls with Turner’s syndrome are at risk.

Clinical Presentation – It may present as:
(1) Thyromegaly with euthyroidism (Asymptomatic goiter)
(2) Toxic thyroiditis
(3) Hypothyroidism with/without thyromegaly.

Cardiac involvement is the most important manifestation of Kawasaki’s disease. 10-40 % of untreated children have evidence of coronary vasculitis within 2 weeks of illness as seen by coronary dilatation or aneurysm on 2D Echo that may lead to myocarditis and on long term early myocardial infarction.

Laboratory investigations show predominance of leukocytosis, thrombocytosis, elevated C-reactive proteins and high ESR.

Treatment consists of intravenous gamma globulin (IVIG) during the acute phase along with high dose aspirin (100mg/kg/day) to prevent coronary vascular damage. IVIg is given as a single dose (2gm/kg) over 10-12 hours. Low dose aspirin (5mg/kg/day) as single daily dose is advocated for continuation phase for 6-8 weeks after the active disease subsides for its anti-thrombotic effects and till coronary lesions resolve. For patients unresponsive to initial treatment with IVIg a retreatment with second dose of IVIg is recommended. It is found that patients with low Hb (Hb< 10gm/dl), leucocytosis and low albumin are at risk of failure of initial IVIg and may require retreatment with second dose of IVIg. For patients with active coronary thrombosis or peripheral artery ischemia, thrombolytic therapy with streptokinase may be required.

Diagnosis

Laboratory investigations: Thyroid function tests (TSH, T4, free T4, T3)
Antithyroid antibodies: AMA, ATG
The presence of euthyroid goiter or hypothyroidism with clinical signs of chronic lymphocytic thyroiditis and positive thyroid antibodies does not warrant further investigation such as radionucleide scan or biopsy of the thyroid gland.

Treatment

If there is hypothyroidism, replacement with thyroxine (50-150 mcg daily) is required. Antibody levels may fluctuate and persist for years. Periodic evaluation including of untreated patients is required as the disease is self-limited. Prominent nodules that persist require evaluation for carcinoma.