First Glance

Hypothyroidism

Hypothyroidism is the most common disorder of thyroid function. It is reported to be more common in women than in men. In hypothyroidism, there is decreased production and secretion of the thyroid hormones by the thyroid gland.

The principal role of the thyroid gland is to regulate tissue metabolism through production of the thyroid hormones thyroxine (T4) and in smaller amounts tri-iodothyronine (T3). In infants and children thyroid hormones are also needed for normal growth and development. Hypothyroidism can be classified on the basis of aetiology, age of onset and on the basis of its severity. The coming pages of this section provide detailed discussion about classification of hypothyroidism.

Aetiology

- Hypothyroidism can be classified on the basis of aetiology as primary, secondary or transient.

a) Primary Hypothyroidism

This is hypothyroidism caused by disorders of the thyroid gland itself.

Causes :

• Destruction of thyroid tissue
• Chronic autoimmune thyroiditis
• goitrous
• atrophic
• Post thyroidectomy
• Post radioactive iodine therapy for thyrotoxicosis
• Neck radiatio
• Iodine deficiency
• Disorders of hormone synthesis-enzyme defects
• Antithyroid agents, lithium, iodine, radiocon-trast dyes containing iodine, amiodarone

The commonest cause of primary hypothyroidism in iodine-sufficient areas is chronic autoimmune thyroiditis and in iodine-deficient areas - iodine deficiency itself.

b) Central / Secondary Hypothyroidism

Decreased thyroid hormone production and secretion by the thyroid gland due to inadequate stimulation by thyroid stimulating hormone (TSH) because of pituitary or hypothalamic disorders.

Causes :

• Pituitary disorders - decreased TSH
• Hypothalamic disorders-decreased Thyrotropin Releasing Hormone (TRH)

• Silent thyroiditis
• Postpartum thyroiditis
• Subacute thyroiditis
• After withdrawal of thyroid hormone therapy in euthyroid patients

Age of onset

Hypothyroidism is also classified on the basis of age of onset as

• Congenital hypothyroidism
• Cretinism
• Juvenile hypothyroidism
• Adolescent hypothyroidism
• Adult hypothyroidism.