Women-specific reference ranges for serum TSH in Liguria: the impact of age and year of collection in a single-center cross-sectional study

Background TSH is the first-line test of thyroid function, and the normal TSH references provided by manufacturers are generally used in diagnoses. In the age of gender medicine, however, there is a need to refine normal TSH ranges. Aim The aim of this study was to construct a normal TSH range in women living in our district. The data were collected in a secondary-level centre located in Savona (Liguria, Italy). Methods From 2003 to 2022, 6227 medical records from women undergoing their first endocrinological examination were anonymously evaluated. After the application of exclusion criteria, statistical analysis was anonymously performed on a sample of 2597 medical records. Results The pooled median 2.5th and 97.5th percentiles of TSH provided by manufacturers were 0.20 mIU/l and 5.64 mIU/l, respectively. In the study population, median (2.5th - 97.5th percentiles) TSH was 1.70 mIU/l (0.37-6.95 mIU/l). TSH and patient age did not vary significantly over the years (2003-2022). A slight negative correlation was found between TSH and age (P = 0.05). On stratifying the sample into three age-groups (18-44 years, N = 1200; 45-64 years N = 934; >= 65 years, N = 463), TSH was 1.75 mIU/l (0.49-5.94 mIU/l), 1.70 mIU/l (0.30-6.89 mIU/l) and 1.64 mIU/l (0.30-7.69 mIU/l), respectively. When TSH was evaluated according to the age-related range instead of the pooled range reported by manufacturers, the number of women aged 18-44 years considered to have sub-clinical hyperthyroidism increased slightly (P = 0.02) and the number of women in the 45-64-year and >= 65-year age-groups considered to have sub-clinical hypothyroidism decreased significantly (P = 0.05 and P < 0.001). Conclusions This is the first study in Liguria aimed at establishing new age-specific reference values for TSH in women. Based on a large number of data, this new age-related range could be more extensively employed in order to improve diagnosis. The main result of implementing age-related normal TSH levels between the 2.5th and 97.5th percentiles seems to be both a slight increase in 18-44-year-old women and a significant reduction in > 45-year-old women in whom sub-clinical hyperthyroidism or hypothyroidism, respectively, should be promptly treated.