Understanding Autoimmune Thyroiditis: Types and Implications

Autoimmune thyroiditis is a complex condition categorized primarily into two types: Hashimoto thyroiditis and atrophic thyroiditis. The key distinguishing feature is the presence or absence of a goitre, an abnormal enlargement of the thyroid gland. Hashimoto thyroiditis is characterized by this enlargement, while atrophic thyroiditis, also known as primary myxoedema, occurs without it. Despite these differences, the underlying disease mechanisms are fundamentally similar, and both can exhibit overlaps with hyperthyroidism, particularly in cases related to Graves' disease.

The causes of hypothyroidism can be classified into primary and secondary/tertiary categories. Primary causes include autoimmune conditions like Hashimoto thyroiditis as well as iodine deficiency, drug-induced effects (such as those from lithium), and congenital conditions. Notably, atrophic thyroiditis and other forms of hypothyroidism can occur without goitre, resulting from factors like radioiodine treatment or surgical interventions on the thyroid gland.

Patients with autoimmune thyroiditis often experience significant lymphocytic infiltration of the thyroid gland, leading to the production of auto-antibodies that block the thyroid-stimulating hormone (TSH) receptor. This process contributes to the progressive destruction of thyroid follicular cells, ultimately resulting in hypothyroidism. Riedel thyroiditis is a particularly rare form characterized by progressive fibrosis that creates a hard goitre, differentiating it from more common types of autoimmune thyroiditis.

Congenital hypothyroidism, which affects approximately 1 in 4,000 births, can arise from failures in thyroid gland formation, migration, or hormone biosynthesis. This condition is typically identified early in life, with pediatric endocrinologists using dried blood spots to measure TSH levels in newborns. Early detection is crucial to prevent long-term developmental issues associated with untreated congenital hypothyroidism.

Moreover, autoimmune thyroid diseases often share a genetic predisposition, placing patients at a higher risk for other autoimmune conditions. This relationship is particularly evident in type 2 autoimmune polyglandular syndrome, where individuals may also develop other disorders such as Addison's disease, type 1 diabetes, or pernicious anaemia.

Understanding the complexities of autoimmune thyroiditis, its causes, and associated conditions is essential for effective diagnosis and management, paving the way for better patient outcomes.