Causes of Hypothyroidism
The most common cause of hypothyroidism in the United States is chronic autoimmune thyroiditis (Hashimoto's disease), while worldwide, iodine deficiency remains the leading cause. 1
Primary Causes of Hypothyroidism
Hypothyroidism typically results from a primary thyroid gland abnormality, with only a small percentage of cases stemming from hypothalamic or pituitary dysfunction. The major causes include:
Autoimmune Thyroid Disease
- Hashimoto's thyroiditis: The predominant cause in industrialized nations 1
- Characterized by lymphocytic infiltration of the thyroid gland
- Gradual destruction and fibrous replacement of thyroid tissue 2
- Presence of anti-thyroid peroxidase (anti-TPO) antibodies
Iodine-Related Causes
- Iodine deficiency: Most common cause globally 1
- Excess iodine intake: Paradoxically can cause hypothyroidism in susceptible individuals
- Iodine-containing medications: Such as amiodarone 1
Iatrogenic Causes
- Previous treatment for hyperthyroidism:
- Radiation therapy: External-beam radiation in the head and neck area 1
Medication-Induced Hypothyroidism
- Lithium: Commonly used in bipolar disorder 3
- Immune checkpoint inhibitors: Used in cancer treatment 3
- Other medications that can affect thyroid function
Other Causes
- Subacute thyroiditis: Inflammatory condition that can lead to transient or permanent hypothyroidism 1
- Congenital hypothyroidism: Present from birth due to thyroid gland developmental issues
- Infiltrative disorders: Such as amyloidosis, sarcoidosis
Risk Factors for Developing Hypothyroidism
Several factors increase the risk of developing hypothyroidism:
- Female sex: Women are more commonly affected than men 1
- Advancing age: Risk increases with age 1
- White race: Higher prevalence in Caucasians 1
- Comorbid autoimmune conditions: Particularly type 1 diabetes 1
- Down syndrome: Higher prevalence of thyroid dysfunction 1
- Family history of thyroid disease: Genetic predisposition 1, 2
- Previous hyperthyroidism: Particularly after treatment 1
- Goiter: Presence of thyroid enlargement 1
Clinical Implications
Untreated hypothyroidism can lead to significant morbidity and mortality:
- Cardiovascular effects: Increased risk of heart failure and atherosclerosis
- Pregnancy complications: Higher risk of preeclampsia, preterm delivery, and low birth weight 1
- Neurological impact: Cognitive impairment, delayed milestones in infants 4
- Metabolic consequences: Weight gain, dyslipidemia 5
- Advanced hypothyroidism: Can progress to myxedema coma, a life-threatening condition 1
Diagnostic Approach
The diagnosis of hypothyroidism is based on:
- TSH testing: Primary screening test, typically elevated in hypothyroidism 1
- Free T4 levels: Usually low or low-normal in overt hypothyroidism 1
- Antibody testing: Anti-TPO antibodies to confirm autoimmune etiology 2
- Thyroid ultrasound: Shows reduced echogenicity in autoimmune thyroiditis 2
Treatment Considerations
The standard treatment for hypothyroidism is levothyroxine (T4) replacement therapy 3. This is particularly important as untreated hypothyroidism can lead to severe complications including myxedema coma in adults and mental retardation in infants 4.
Common Pitfalls in Diagnosis and Management
- Overlooking subclinical hypothyroidism: Defined by elevated TSH with normal free T4
- Misattributing nonspecific symptoms: Fatigue, weight gain, and cold intolerance can have many causes
- Inadequate monitoring: Failure to adjust levothyroxine dosage based on TSH levels
- Ignoring special populations: Pregnancy, elderly, and those with cardiac disease require special consideration
Understanding the diverse causes of hypothyroidism is essential for proper diagnosis and management, ultimately reducing morbidity and mortality associated with this common endocrine disorder.