Drug-induced hypothyroidism

Definition

Drug-induced hypothyroidism involves decreased activity of the thyroid gland caused by an adverse reaction to a medication.

Causes, incidence, and risk factors

Hypothyroidism (underactivity of the thyroid gland) can cause a variety of symptoms that affect the entire body. The body's normal rate of functioning slows, causing mental and physical sluggishness. Symptoms vary from mild to severe.

Drug-induced hypothyroidism may be caused by treatment with lithium or iodides in susceptible people. Drugs used for hyperthyroidism (overactive thyroid) such as PTU and methimazole may cause unintentional hypothyroidism. Unusual causes of drug-induced hypothyroidism include nitroprusside, perchlorate, and sulfonylureas.

The most severe form of hypothyroidism is called myxedema coma, which is a medical emergency. Severe hypothyroidism, however, is seldom drug-related.

Symptoms

Early symptoms:

• Weakness
  
• Fatigue
  
• Cold intolerance
  
• Constipation
  
• Weight gain
  
• Depression
  
• Brittle fingernails
  
• Coarsening and thinning of hair
  
• Dry skin
  
• Puffy eyes
  

Late symptoms:

• Slow speech
  
• Thickening of the skin
  
• Puffy face, hands, and feet
  
• Decreased hearing ability
  
• Thinning of eyebrows
  
• Hoarseness
  
• Menstrual disorders
  

Additional symptoms that may be associated with this disease include joint stiffness.

Signs and tests

A physical examination may reveal an enlarged thyroid gland and delayed deep tendon reflexes. An assessment of vital signs (temperature, pulse, rate of breathing, blood pressure) may reveal slow heart rate, low blood pressure, and low temperature. A chest x-ray may reveal an enlarged heart.

Laboratory tests to determine thyroid function include the following:

• Free T4 test
  
• T3 levels
  
• Serum TSH (thyroid stimulating hormone)