Age-specific reference intervals for thyroid-stimulating hormone and free thyroxine to optimize diagnosis of thyroid disease

Most laboratories do not differentiate reference intervals for thyroid hormones for adults based on age. In a national study we established age-specific reference intervals for TSH and FT4. For children aged 2-12 years, TSH reference intervals were higher than for adults. In adults (female and male), the upper limits of the TSH reference intervals increased from the age of 50-60 years, while the lower limits remained stable until the age of 80. The FT4 reference intervals showed a moderate trend with age. These new age-specific reference intervals led to a decrease in subclinical hypothyroidism diagnoses. For women aged 50-60 from 13.1% to 8.6%; for women aged 90-100, from 22.7% to 8.1%. In men aged 60-70, it decreased from 10.9% to 7.7%, and aged 90-100, from 27.4% to 9.6%. This study confirms the importance of implementing age-specific reference intervals for TSH and FT4 to reduce the number of (unnecessary) diagnoses of subclinical hypothyroidism.