# Activity of Cardiomyocyte Type 3 Deiodinase After Myocardial Infarction Influences Cardiac Recovery in Females

## TL;DR

This study shows that a thyroid hormone-regulating enzyme, DIO3, helps female hearts recover better after a heart attack by limiting harmful hormone levels and protecting mitochondria.

## Contribution

The study reveals a sex-specific role of DIO3 in post-MI cardiac recovery, highlighting a novel therapeutic target for females.

## Key findings

- Female mice lacking D3 showed impaired heart recovery and higher T3 levels after MI.
- DIO3 was selectively upregulated in female human heart tissue with ischemic cardiomyopathy.
- D3 activity in females limits T3-induced metabolic stress and preserves mitochondrial function.

## Abstract

Thyroid hormone (TH) is essential for cardiovascular function, and women are disproportionately affected by TH disorders and experience worse outcomes following myocardial infarction (MI). However, the role of sex-specific TH regulation in post-MI cardiac recovery remains poorly understood. We investigated TH homeostasis and type 3 deiodinase (D3) activity, an enzyme that inactivates TH, in male and female C57BL/6 mice following MI. Using cardiomyocyte-specific D3-deficient (Dio3ΔHeart) mice, we investigated how impaired TH inactivation influences cardiac function and mitochondrial respiration. We also examined DIO3 messenger RNA expression, which encodes the D3 enzyme, in left ventricular (LV) tissue from human donors with nonfailing (NF) hearts or ischemic cardiomyopathy (ICM). Four weeks post MI, wild-type female mice exhibited sustained cardiac D3 activity, which effectively limited 3,5,3′-triiodothyronine (T3) levels in the LV. In contrast, Dio3ΔHeart females, lacking cardiomyocyte D3, showed impaired systolic recovery, elevated LV thyroxine and T3 levels, and reduced fatty acid–supported mitochondrial respiration, effects not observed in Dio3ΔHeart males. Similarly, DIO3 expression was selectively upregulated in LV tissue from women with ICM, but not in men. These findings identify DIO3 as a key protective mechanism in females that limits T3-induced metabolic stress and preserves mitochondrial function after MI, revealing a sex-dependent pathway with therapeutic relevance for cardiac recovery.