Drug Therapy of Hypothyroidism

Introduction

• Definition — Hypothyroidism is the clinical state of thyroid hormone deficiency; the most common disorder of thyroid function. Severe deficiency is termed myxoedema, after the characteristic non-pitting thickening of subcutaneous tissue.
• Therapeutic principle — Unlike hyperthyroidism (which needs synthesis-blocking/ablative drugs), hypothyroidism is treated by thyroid hormone replacement — supplying the deficient hormone to restore physiological levels.
• Prohormone logic — The thyroid secretes predominantly the prohormone T₄ (thyroxine), which is converted peripherally to the active hormone T₃; replacement gives T₄ and lets the body generate T₃ — the basis of levothyroxine monotherapy.
• Drug of choice — Synthetic l-thyroxine (levothyroxine, T₄) is the preparation of choice for replacement in deficiency states.
• Aetiology drives the monitoring target — Primary (gland failure — iodine deficiency worldwide; Hashimoto autoimmune thyroiditis with anti-TPO antibodies in iodine-sufficient areas) → TSH high, monitor by TSH. Central (secondary pituitary / tertiary hypothalamic) → only free T₄ low, TSH unreliable, monitor by free T₄. Congenital ("cretinism") is a major preventable cause of intellectual disability. Drug-induced causes: lithium, amiodarone, iodides, radioiodine.