Objective: Thyroid function, influenced by age and sex, provides important epidemiological information when evaluated at the population level. This study aimed to investigate age- and sex-related differences in thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4), and anti-thyroid peroxidase (anti-TPO) antibody positivity in a Turkish population using a large laboratory database.

Methods: In this retrospective, laboratory-based study, data from 100,749 individuals tested at the Karabük Public Health Laboratory between July 2022 and August 2025 were analyzed. Individuals with missing demographic or laboratory data were excluded. TSH, fT3, fT4, and anti-TPO antibody levels were measured using a chemiluminescent immunoassay method (Cobas e601 analyzer). Participants were categorized into eight age groups: 0–5 years, 6–12 years, 13–17 years, 18–29 years, 30–44 years, 45–59 years, 60–74 years, and ≥75 years.

Results: TSH and fT3 levels decreased progressively with advancing age, whereas fT4 levels showed minor variations across age groups (p<0.001). The prevalence of elevated TSH, subclinical hypothyroidism, and overt hypothyroidism increased with age and was higher in women than in men (p<0.001). Spearman correlation analysis demonstrated significant associations between age and thyroid parameters, except for fT4, with the strongest correlation observed between age and fT3 (r = -0.493, p<0.001).

Conclusion: Clear age- and sex-related variations in thyroid function were observed, highlighting the importance of demographic context in interpreting thyroid tests, while patterns such as low TSH with low fT3 were interpreted cautiously and may reflect non-thyroidal illness rather than true thyroid dysfunction.