Serum Thyrotropin and Triiodothyronine Levels in Levothyroxine-treated Patients

CONTEXT

Small adjustments in levothyroxine (LT4) dose do not appear to provide clinical benefit despite changes in thyrotropin (TSH) levels within the reference range. We hypothesize that the accompanying changes in serum total triiodothyronine (T3) levels do not reflect the magnitude of the changes in serum TSH.

OBJECTIVE

This work aims to characterize the relationships of serum free thyroxine (FT4) vs T3, FT4 vs TSH, and FT4 vs the T3/FT4 ratio.

METHODS

This cross-sectional, observational study comprised 9850 participants aged 18 years and older treated with LT4 from a large clinical database from January 1, 2009, to December 31, 2019. Patients had been treated with LT4, subdivided by serum FT4 level. Main outcome measures included model fitting of the relationships between serum FT4 vs TSH, FT4 vs T3, and FT4 vs T3/FT4. Mean and median values of TSH, T3, and T3/FT4 were calculated.

RESULTS

The relationships T3 vs FT4 and TSH vs FT4 were both complex and best represented by distinct, segmented regression models. Increasing FT4 levels were linearly associated with T3 levels until an inflection point at an FT4 level of 0.7 ng/dL, after which a flattening of the slope was observed following a convex quadratic curve. In contrast, increasing FT4 levels were associated with steep declines in TSH following 2 negative sigmoid curves. The FT4 vs T3/FT4 relationship was fit to an asymptotic regression curve supporting less T4 to T3 activation at higher FT4 levels.

CONCLUSION

In LT4-treated patients, the relationships between serum FT4 vs TSH and FT4 vs T3 across a range of FT4 levels are disproportionate. As a result, dose changes in LT4 that robustly modify serum FT4 and TSH values may only minimally affect serum T3 levels and result in no significant clinical benefit.

OR19-2 Big Data Strategy Used to Draw TSH Reference Values for Elderly Population

Introduction: Determination of reference ranges for thyroid function tests, such as TSH, are not a simple task. There are several biological and individual aspects to be considered, one of the most important is age, particularly for the extremes, children and the elderly population. Several studies demonstrated a progressive increase in age-related TSH in older adults. Therefore, the aim of this study is to establish specific reference values for Brazilian population over 60 years old using Big Data strategy. Methods: Data from 10.43 million TSH tests performed in the same method were retrospectively analyzed. All the samples are from the same clinical laboratory, with samples from several regions of Brazil. The samples were selected using the following filters: free T4 concentration within the reference values (RV), absence of anti-thyroid antibodies and of medication use. Groups were divided by age from 18 to 59 years old (under-60), and over 60 up to 107 years old (over-60). Statistical analyzes were performed using R software and EP Evaluator. Both R package ‘boot’ and EP Evaluator nonparametric (CLSI C28-A) were used to estimate the 97.5% confidence interval (IC). Results: After filtering the samples, 306,289 were selected. Mean TSH was similar in both sex. However, there were differences between the two age-groups, in under-60 group, the lower RV was 0.59 mUI/L (95% IC = 0.57, 0.59) and the upper RV was 6.00 mUI/L (95% IC = 5.94, 6.08). Interestingly, in over-60 group the lower RV was 0.36 mUI/L (95% IC = 0.34, 0.38) and the upper RV was 9.38 mUI/L (95% IC = 8.86, 9.44). Furthermore, this upper limit of reference range increases within the older group with TSH of 7.4 mUI/L from 60 to 69, 9.60 mUI/L from 70 to 79 and 12.30 mUI/L above 80 years old. Conclusion: Big data strategy could be a useful tool to reach agreeable population reference values. In particular, for TSH, the higher upper reference limits for adults over 60 years old would be important to avoid overdiagnosis and over-treatment of subclinical hypothyroidism in this population.

PSA kinetics before 40 years of age

Purpose:

The baseline PSA has been proposed as a possible marker for prostate cancer. The PSA determination before 40 years seems interesting because it not suffers yet the drawbacks related to more advanced ages. Considering the scarcity of data on this topic, an analysis of PSA kinetics in this period seems interesting.

Materials and Methods:

A retrospective assay in a database of a private diagnostic center was performed from 2003 to 2016. All subjects with a PSA before 40 years were included.

Results:

92995 patients performed PSA between the ages of 21 – 39. The mean value ranged from 0.66 ng / mL (at age 22) to 0.76 ng / mL (at age 39) and the overall mean was 0.73 ng / mL. As for outliers, 3783 individuals presented a baseline PSA > 1.6 ng / mL (p95). A linear regression model showed that each year there is a PSA increase of 0.0055 ng / mL (β = 0.0055; r² = 0.0020; p < 0.001). A plateau in PSA between 23 and 32 years was found and there were only minimal variations among the ages regardless of the evaluated percentile.

Conclusion:

It was demonstrated that PSA kinetics before 40 years is a very slow and progressive phenomenon regardless of the assessed percentile. Considering our results, it could be suggested that any PSA performed in this period could represent the baseline value without significant distortions.