Hypothyroidism

Definition

Hypothyroidism is a clinical syndrome caused by insufficient thyroid hormone production or action, leading to a generalized slowing of metabolic processes. It may be primary (thyroid gland dysfunction) or secondary (pituitary or hypothalamic disease).

Epidemiology

• Affects 1–2% of the population, more common in women and older adults
• Primary hypothyroidism (Hashimoto’s thyroiditis) is the most common form
• Congenital hypothyroidism occurs in 1 in 2000–4000 live births
• Secondary (central) hypothyroidism is rare (<1% of cases)

Etiology

• Primary: autoimmune (Hashimoto’s thyroiditis), iodine deficiency/excess, post-thyroidectomy, radioactive iodine therapy, congenital thyroid agenesis or dysgenesis
• Secondary: pituitary disease (tumor, surgery, irradiation), hypothalamic disease
• Medications: amiodarone, lithium, interferon-alpha
• Transient: thyroiditis (subacute, postpartum)

Pathophysiology

• Thyroid hormone (T3, T4) deficiency → decreased basal metabolic rate → weight gain, cold intolerance, fatigue
• Accumulation of mucopolysaccharides in tissues → myxedema, puffiness, hoarseness
• Impaired cardiovascular, GI, neuromuscular, and reproductive functions
• Feedback: low T3/T4 → increased TSH in primary hypothyroidism; low/normal TSH in secondary hypothyroidism