Hypothyroidism

Sub-section: Thyroid Section: Endocrine

Definition

• Decreased thyroid function

Overview

• Most commonly primary
• Can also rarely be due to peripheral resistance to thyroid hormones
• Most common cause in developing countries
  • Iodine Deficiency
• Most common causes in developed countries
  • Hashimoto thyroiditis
  • Radiation induced
  • Surgical removal
  • Medications
• Diagnosis
  • If clinical suspicion, confirm with TSH levels and fT4 levels
• Treatment
  • Levothyroxine (PO, IM, IV) safe and effective

Iodine Deficiency

• Leads to chronic anatomic and metabolic alterations
  • Chronic decrease in T3 and T4 production
  • Increasing thyroid clearance of iodine and decreased renal excretion
• Body reacts by chronic preferential production of T3 (over T4)
  • Also enhanced peripheral conversion of T4 to T3
  • Avoids clinical hypothyroidism
    • Low fT4, high TSH, normal or increased T3
• In most severe cases, decrease in both T3 and T4 (and TSH increase)
  • Can lead to Endemic Goitre
  • Severe in-utero deficiency can lead to endemic Cretinism
  • Neurologic impairment, stunted growth, mental deficiency, and overt hypothyroidism

Endemic Goitre

• Diffuse enlargement of thyroid in response to physiologic changes of iodine deficiency
  • Hypertrophy of thyroid follicles
    • Reduction of follicular spaces
  • If severe follicles become inactive
    • Distend with colloid
  • Focal areas of nodular hyperplasia may develop
    • Can form nodules, some autonomous hot nodules
    • Can get scarring, necrosis, haemorrhage
      • All contribute to goitre, often asymmetric
• Most common in Southeast Asia
  • India, Indonesia, China
  • Mild in Italy, Hungary, Poland, Spain
• In areas where most severe, can get symptoms at early age
  • Greatest prevalence in later childhood years
  • Peaks at puberty
  • Prevalence decreases in adulthood, remains slightly higher in women

Post-Radiation Hypothyroidism

• Usually planned hypothyroidism as a result of rx for hyperthyroidism with I-131
  • Multinodular goitre
  • Grave’s
• Need to monitor so do not become clinically hypothyroid
• External beam radiation to mediastinum may also cause this
  • Lymphoma or head/neck Ca

Post-Surgical Hypothyroidism

• Caused by subtotal/total thyroidectomy
• More aggressive surgery = Higher risk of complications
• Need to supplement with thyroxine and monitor levels

Pharmacologic Hypothyroidism

• Antithyroid drugs
  • Methimazole/Carbimazole and PTU if given in excess
  • Monitoring mandatory
• Amiodarone, Lithium, Interferon-α, IL-2, antineoplastic drugs
  • Mostly interfere with hormone release from the gland or direct toxicity to thyroid itself
  • Amiodarone
    • Iodine rich (50-100x daily intake)
    • Can cause hyperthyroidism (more common in iodine deficient areas)
    • Amiodarone induced thyrotoxicosis difficult to treat
    • Can also cause hypothyroidism (more common in areas not iodine deficient)
    • Also inhibits peripheral conversion of T4 > T3
• Lithium (for BPAD)
  • Inhibits cAMP pathway of thyroid hormone production
  • Pts with Hashimoto thyroiditis more vulnerable to hypothyroidism from above drugs