# Nonlinear Association Between THs, TSH, and HbA1c in Patients With Type 2 Diabetes Mellitus: A Retrospective Study

## TL;DR

This study found nonlinear relationships between thyroid hormones and blood sugar levels in patients with type 2 diabetes.

## Contribution

The study reveals novel nonlinear associations between thyroid hormones (FT3, FT4, TSH) and HbA1c in type 2 diabetes patients.

## Key findings

- FT4 shows a significant nonlinear relationship with HbA1c, with an inflection point at 14.82 pmol/L.
- TSH demonstrates a nonlinear association with HbA1c, with an inflection point at 5.53 mIU/L.
- Below the TSH threshold, higher TSH levels are inversely associated with HbA1c.

## Abstract

This study aimed to investigate the potential non‐linear relationships between thyroid hormones and thyroid‐stimulating hormone levels on glycemic control levels.

A retrospective analysis of electronic medical records was performed on patients with T2DM who received treatment at a tertiary care hospital in Chengdu, Sichuan Province, between 2018 and 2023. RCS regression and threshold effect analyses were employed to assess potential nonlinear associations among THs, TSH, and glycemic control.

Data from a total of 1413 patients were included in the analysis. RCS regression revealed a significant non‐linear association between FT3 and HbA1c (p for nonlinearity < 0.05). Threshold effect analysis demonstrated no statistically significant inflection point for FT3 in relation to HbA1c (FT3 < 5.92 pmol/L, β = 0.008, 95% CI (−0.122, 0.281); FT3 > 5.92 pmol/L, β = −0.109, 95% CI (−0.261, 0.042)). FT4 exhibited a significant non‐linear relationship with HbA1c (p for nonlinearity < 0.05), identifying an inflection point at 14.82 pmol/L. Below this threshold, each 1 pmol/L increase in FT4 was associated with a 0.263% elevation in HbA1c (β = 0.263, 95% CI 0.189–0.337). Similarly, TSH demonstrated a non‐linear association with HbA1c (p for nonlinearity < 0.05), with an inflection point identified at 5.53 mIU/L. When TSH was below 5.53 mIU/L, each 1 mIU/L increase was associated with a 0.179% reduction of HbA1c (β = −0.179, 95% CI −0.281 to −0.076).

Nonlinear associations were observed between thyroid function markers (FT3, FT4, and TSH) and HbA1c levels in patients with T2DM. These findings provide novel evidence for understanding thyroid‐glucose metabolic interactions.

We used restricted cubic spline modeling and combined it with threshold effects analysis to explore the non‐linear relationship between thyroid hormone, thyroid‐stimulating hormone, and blood glucose levels in patients with T2DM.Non‐linear associations between FT3, FT4, TSH, and glycated hemoglobin in patients with T2DM.

We used restricted cubic spline modeling and combined it with threshold effects analysis to explore the non‐linear relationship between thyroid hormone, thyroid‐stimulating hormone, and blood glucose levels in patients with T2DM.

Non‐linear associations between FT3, FT4, TSH, and glycated hemoglobin in patients with T2DM.

Non‐linear Association: FT4 and TSH display significant non‐linear relationships with HbA1c in T2DM patients. FT4 Threshold (14.82 pmol/L): Below this inflection point, FT4 is positively associated with HbA1c. TSH Threshold (5.53 mIU/L): Below this inflection point, TSH is inversely associated with HbA1c. Provides novel evidence regarding the interaction between thyroid function and glucose metabolism.