[New normal ranges for TSH: when to treat?]

Area of Science:

• Endocrinology
• Biochemistry
• Internal Medicine

Context:

• Subclinical thyroid dysfunction is defined by thyroid-stimulating hormone (TSH) levels outside the established laboratory norm, while circulating thyroid hormone concentrations remain within normal limits.
• Recent population studies suggest a narrower TSH reference range (0.3–2.5 mU/l).
• TSH levels can be influenced by various endogenous and exogenous factors, including laboratory methodologies.

Purpose:

• To evaluate the clinical significance of subclinical thyroid dysfunction based on updated TSH reference ranges.
• To determine the appropriateness of therapeutic intervention for subclinical hypothyroidism.
• To assess the association between subclinical hyperthyroidism and adverse health outcomes.

Summary:

• Subclinical thyroid dysfunction is diagnosed biochemically by deviations in TSH levels from the laboratory norm, with normal thyroid hormone levels.
• Therapeutic intervention for subclinical hypothyroidism is not recommended when TSH levels are below 4.5–5 mU/l, given the absence of data indicating increased morbidity.
• Conversely, subtle TSH alterations in subclinical hyperthyroidism are linked to a notably elevated risk of conditions such as atrial fibrillation.

Impact:

• This research clarifies the diagnostic criteria and management strategies for subclinical thyroid dysfunction.
• It highlights the importance of considering updated TSH reference ranges in clinical practice.
• The findings underscore the significant health risks associated with subclinical hyperthyroidism, particularly cardiovascular complications like atrial fibrillation.