Subclinical hypothyroidism: a historical view and shifting prevalence

Area of Science:

• Endocrinology
• Thyroid Disorders
• Clinical Practice

Background:

• Subclinical hypothyroidism (SCH) diagnosis and treatment present challenges in clinical practice.
• Variability in upper limits of normal (ULN) for thyroid-stimulating hormone (TSH) complicates accurate diagnosis.
• Different definitions of SCH significantly impact reported prevalence rates.

Purpose of the Study:

• To review various definitions of SCH.
• To analyze the impact of these definitions on reported SCH prevalence.
• To discuss the implications for clinical diagnosis and treatment.

Main Methods:

• Systematic literature search on PubMed using keywords 'subclinical hypothyroidism,' 'prevalence,' and 'TSH'.
• Inclusion of English-language articles reporting SCH prevalence in relation to TSH ULN in human studies.
• Selection of relevant articles and related literature for comprehensive review.

Main Results:

• Reported SCH prevalence varies widely (0.4-16.9%) based on sex, age, race/ethnicity, and geography.
• Higher SCH rates observed in women and older individuals.
• Age-related increase in TSH ULN suggests potential overestimation of SCH in the elderly; elevated TSH with antibodies indicates increased risk of overt hypothyroidism.

Conclusions:

• The inconsistent ULN for TSH makes true SCH prevalence difficult to ascertain and impacts patient labeling and treatment.
• Age-adjusted definitions may improve prevalence assessment.
• SCH diagnosis does not always warrant treatment, particularly if TSH elevation is transient and no other risk factors are present.