What is subclinical hypothyroidism?

Understanding Subclinical Hypothyroidism

Subclinical hypothyroidism is defined as an elevated thyroid stimulating hormone (TSH) level above the normal reference range (4.5 mIU/L) with normal free thyroxine (FT4) and triiodothyronine (T3) levels. 1 This condition represents an early stage of thyroid dysfunction that may or may not progress to overt hypothyroidism.

Key Characteristics

• Subclinical hypothyroidism affects approximately 4-8.5% of the general population without known thyroid disease 1
• It is more common in women, especially those over 60 years old, where prevalence can reach up to 20% 1
• About 75% of patients with elevated TSH have values lower than 10 mIU/L 1
• The normal reference range for TSH is considered to be 0.45-4.5 mIU/L 1

Risk Factors

• Age (increasing prevalence with older age) 1
• Female gender (more common in women than men) 1
• Family history of thyroid disease 1
• Previous hyperthyroidism 1
• Type 1 diabetes mellitus 1
• Previous head and neck radiation treatment 1
• Presence of antithyroid antibodies (increases risk of progression) 1, 2

Progression and Natural History

• Approximately 2-5% of patients with subclinical hypothyroidism progress to overt hypothyroidism each year 1, 2
• The risk of progression is higher with:
  • Higher baseline TSH levels 1
  • Presence of antithyroid antibodies 1
• In about 5% of cases, TSH levels normalize spontaneously within a year 1
• In the remaining cases, TSH typically remains elevated 1

Potential Health Consequences

• Cardiac dysfunction and possible increased risk of cardiovascular events 1
• Elevated total and LDL cholesterol levels 1
• Possible systemic hypothyroid symptoms (though often minimal) 1
• Neuropsychiatric symptoms in some cases 1
• Increased risk of progression to overt hypothyroidism 1, 2

Treatment Considerations

• Treatment is recommended for all patients with TSH >10 mIU/L 2
• For TSH between 4.5-10 mIU/L, treatment should be considered in:
  • Symptomatic patients 2, 3
  • Pregnant women or those planning pregnancy 2
  • Patients with infertility 2
  • Patients with goiter or positive anti-thyroid peroxidase antibodies 2, 4
  • Younger patients (<65 years) with cardiovascular risk factors 3
• Treatment may be avoided in the very elderly (>85 years) with mild elevation (TSH <10 mIU/L) 2

Common Pitfalls in Diagnosis

• Other conditions can cause temporary TSH elevation and should be excluded:
  • Recent adjustments in levothyroxine dosage 1
  • Recovery from severe illness 1
  • Recovery from thyroiditis 1
  • Untreated adrenal insufficiency 1
  • Laboratory interference from heterophilic antibodies 1
• TSH values can be affected by medications like dopamine, glucocorticoids, and dobutamine 1
• Normal TSH values vary slightly with age, sex, and ethnicity, but these differences are generally not clinically significant 1

Understanding subclinical hypothyroidism is important because it represents a common condition that may impact quality of life and carries a risk of progression to overt hypothyroidism, which has more significant health consequences if left untreated.