Reference Intervals of Thyrotropin, Thyroid Hormones, and Thyroid Autoantibodies in Adult and Older Individuals According to Iodine Status.

Background-aim: Several factors, including ethnicity, age, iodine status, and assay method, can influence thyroid test results. This study aimed to establish reference intervals (RIs) for thyroid parameters in adults and older individuals, considering their iodine statuses.

Methods: A cross-sectional study at a single tertiary center was conducted. Participants underwent interviews, physical examinations, thyroid ultrasounds, thyroid autoantibody testing, and a spot urine iodine concentration analysis. The included participants were grouped into adult (age 18 - 59) and older (age ≥60) groups. The studies for 2.5th-97.5th values of thyroid parameters were committed to establishing RIs.

Results: A total of 357 individuals were screened, with 216 (112 adults, 54% women, 104 older, 50% women) were included in the analysis. The RIs for thyroid stimulating hormone (TSH) were as follows: 0.39 - 4.17 mIU/L for the overall group, 0.35 - 3.98 mIU/L in the adult group, and 0.42 - 4.83 mIU/L in the older group. The prevalence of adequate iodine intake (urine iodine level ≥ 200 μg/L) was 62.2% (186/299). Though RIs for TSH, Free T4, and Free T3 were slightly higher in the adequate iodine intake group, no statistically significant differences were noted. Positive anti-thyroglobulin antibodies were more prevalent in older participants (27.3% VS. 21.2%), as were anti-thyroid peroxidase antibodies (24.2% VS. 13.9%).

Conclusion: Older individuals exhibited significantly higher TSH levels and lower FT3/Total T3 levels, while FT4/Total T4 remained comparable to adults. All thyroid parameters and thyroid autoantibody levels showed no statistically significant differences between those with adequate iodine intake and those with iodine deficiency.