50. Hypothyroidism - classification, etiology, pathogenesis, clinical manifestations.

Hypothyroidism - classification, etiology, pathogenesis, and clinical manifestations

Hypothyroidism is a condition caused by deficient production or action of thyroid hormones (T3 and T4), leading to a generalized reduction in metabolic processes.

Classification

1. 1 According to level of defect

1. 2 According to onset

Etiology

• Primary hypothyroidism (most common)
• Chronic autoimmune thyroiditis (Hashimoto thyroiditis) — most common cause in iodine-sufficient regions.
• Iodine deficiency —most common worldwide.
• Post-thyroidectomy or radioiodine therapy.
• Drugs: lithium, amiodarone, antithyroid drugs.
• Congenital defects: thyroid dysgenesis or dyshormonogenesis.
• Secondary hypothyroidism
• Pituitary tumors or surgery.
• Sheehan syndrome (postpartum pituitary necrosis).
• Tertiary hypothyroidism
• Hypothalamic tumors or damage.

Pathogenesis

1. Reduced T3 and T4 production (thyroid failure or central suppression).
2. ↓ Thyroid hormones → ↓ basal metabolic rate.
3. ↓ Thermogenesis → cold intolerance.
4. Accumulation of mucopolysaccharides (glycosaminoglycans) in interstitial tissues → myxedema (non-pitting edema).
5. Increased TSH in primary hypothyroidism (due to loss of negative feedback).

Clinical manifestations

• General metabolic signs
• Fatigue, lethargy.
• Weight gain despite poor appetite.
• Cold intolerance.
• Skin and hair
• Dry, coarse skin.
• Brittle hair, hair loss.
• Non-pitting edema (myxedema), especially face and periorbital area.
• Cardiovascular
• Bradycardia.
• Decreased cardiac output → fatigue, exercise intolerance.
• Possible pericardial effusion.
• Neuromuscular
• Muscle weakness, cramps.
• Delayed deep tendon reflex relaxation ("hung-up reflexes").
• Gastrointestinal
• Constipation.
• Reduced motility.
• Reproductive
• Women: menorrhagia (heavy periods), oligomenorrhea, infertility.
• Men: decreased libido, erectile dysfunction.
• Neuropsychiatric
• Depression, slow thought processes ("mental sluggishness").
• In severe cases: psychosis ("myxedema madness").
• Severe form
• Myxedema coma:
• Life-threatening severe hypothyroidism.
• Hypothermia, hypoventilation, hypotension, hypoglycemia, bradycardia, altered mental status.
• Precipitated by infections, trauma, cold exposure.
• Congenital hypothyroidism (cretinism)
• Caused by thyroid dysgenesis, maternal iodine deficiency.
• Features:
• Severe mental retardation.
• Short stature, coarse facial features.
• Umbilical hernia.
• Enlarged tongue (macroglossia).