ODP460 Age- and sex- specific reference ranges for serum thyrotropin (TSH), free T4 and free T3 levels: Significant effects on the definition of subclinical dysfunction

INTRODUCTION: Consistent reference ranges of serum TSH, FT4 and FT3 levels without consideration of age and sex have been used to evaluate thyroid function. However, based on the recent development of highly sensitive assays of these hormones, subtle differences significantly affect the diagnosis, especially when diagnosing subclinical dysfunction, which affects more than 5% of the population. In the present study, we investigated the age- and sex- differences in these hormone levels using two different kits. METHODS: A total of 14,860 subjects for the Siemens kit and 6,604 subjects for the Abbot kit who underwent annual health check-ups at Takasaki Hidaka Hospital were included. Exclusion criteria comprised a history of thyroid disease, steroid use, renal failure or liver cirrhosis. All samples were measured at 8–9: 00 AM after overnight fasting. RESULTS: Regarding serum TSH levels in men, the median of subjects in their 30's was 1.4 mIU/L, the 2.5th percentile was 0.4, and the 97.5th percentile was 3.7 with the Siemens kit. These values in subjects in their 60's were 1.6, 0.5, and 5.7, respectively. They gradually but significantly increased with age based on both kits. Similarly in women, the serum TSH levels gradually increased, particularly with the Siemens kit, and the levels in women were higher than those in men. The median serum FT4 level in men in their 30's was 1.3 ng/dL (1. 0-1.7) and that in those in their 60's was 1.2 (1. 0-1.6) with the Siemens kit. The levels in men gradually but significantly decreased with age, but those in women were lower than those in men and remained consistent with age. Regarding serum FT3 levels in men, they were significantly higher than those in similarly aged women, and also gradually but significantly decreased with age; men in their 30's: 3.37 pg/mL (2.68-3.99), 60's: 3.15 (2.46-3.76) with the Abbot kit. DISCUSSION: The serum TSH level is highly sensitive to slight changes in the serum level of thyroid hormones even within the normal ranges; therefore, we diagnose subclinical hypothyroidism and hyperthyroidism using serum TSH levels. Many investigators have discussed the effects of subclinical dysfunction on lipid metabolism, osteoporosis, and cognitive mental diseases, and medications to normalize thyroid hormone levels. As there are significant differences in serum TSH, FT4, and FT3 levels according to age and sex, the age- and sex- dependent normal ranges should be used to precisely evaluate thyroid function and decide clinical therapies. Presentation: No date and time listed