Thyroid Hormones in CKD (THYROID-CKD)

Chronic kidney disease (CKD) is frequently associated with alterations in thyroid hormone homeostasis, commonly referred to as Non-Thyroidal Illness Syndrome (NTIS) or "low T3 syndrome" . This condition is typically characterized by reduced circulating levels of free triiodothyronine (fT3) in the presence of normal or slightly decreased Thyroid-Stimulating Hormone (TSH) and free thyroxine (fT4), and has been associated with inflammation, protein-energy wasting, and the severity of renal dysfunction.

While these hormonal changes are generally interpreted as an adaptive metabolic response to chronic illness, accumulating evidence suggests that alterations in thyroid hormone metabolism in CKD may reflect more complex pathophysiological mechanisms, including impaired peripheral deiodination, mitochondrial dysfunction, chronic inflammation, and altered availability of enzymatic cofactors.

In this context, reverse triiodothyronine (rT3), a thyroid hormone metabolite generated through peripheral deiodination of thyroxine (T4), has been proposed as a potential marker of altered thyroid hormone metabolism. Although rT3 is not routinely used in clinical practice, it may provide additional insights into the balance between activating and inactivating pathways of thyroid hormone metabolism, particularly when interpreted in combination with fT3 (e.g., rT3/fT3 ratio).

Recent findings from the REORIENTED study conducted in a well-characterized cohort of patients with autosomal dominant polycystic kidney disease (ADPKD), identified a distinct thyroid hormone profile characterized by increased rT3 levels, reduced fT3 concentrations, and a significant association between both fT3 and rT3 levels and renal function, measured as estimated glomerular filtration rate (eGFR). These relationships were particularly strong in patients with moderate to severe kidney dysfunction, who exhibit an increased rT3/fT3 ratio compared to patients with normal to mild decrease in eGFR, probably due to an increased conversion of T4 into rT3 at the expense of the production of fT3.

These observations raise the hypothesis that ADPKD may be associated with disease-specific alterations in thyroid hormone profile, potentially reflecting unique features of cystic kidney disease. However, it remains unclear whether this hormonal pattern is specific to ADPKD or rather represents a general feature of advanced CKD, independent of the etiology.

To address this question, a comparative analysis with non-ADPKD CKD patients with similar kidney function is required. By leveraging biobank-stored samples and clinical data from well-characterized CKD cohorts, it is possible to investigate whether the alterations observed in ADPKD are also present in other forms of advanced renal disease.

If a similar pattern of increased rT3 and reduced fT3 (or altered rT3/fT3 ratio) is observed in non-ADPKD CKD patients, this would support the hypothesis that these changes primarily reflect reduced kidney function and systemic illness. Conversely, if such alterations are attenuated or absent, this would suggest that ADPKD is characterized by a distinct modulation of thyroid hormone metabolism, supporting a disease-specific pathophysiological mechanism.

In this perspective, the present biobank-based observational comparative study was designed to characterize circulating thyroid hormone profiles in severe non-ADPKD CKD and to compare them with those observed in a matched subset of ADPKD patients from the REORIENTED cohort, with particular emphasis on rT3 and the rT3/fT3 ratio as biomarkers of altered peripheral thyroid hormone metabolism.

In this context, if the observed thyroid hormone alterations are hypothesized to reflect mechanisms related to impaired kidney dysfunction rather than ADPKD itself, the ADAPT cohort-a prospective, randomized, double-blind, crossover, placebo-controlled trial published in 2025, which demonstrated beneficial effects of dapagliflozin on hyperfiltration and proteinuria in non-diabetic patients with advanced CKD-may provide a unique opportunity to further investigate thyroid hormone metabolism in advanced kidney disease. Specifically, the availability of biological samples will enable the assessment of serum and urinary thyroid hormone profiles before and after SGLT2 inhibitor treatment, to explore whether these alterations may be modulated by dapagliflozin.