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.