Impact of Continuous Veno-Venous Hemodiafiltration on Thyroid Homeostasis in Critically Ill Patients

Background: Patients in Intensive Care Units (ICUs) often develop non-thyroidal illness syndrome. Potentially, thyroid hormones may be removed during continuous veno-venous hemodiafiltration (CVVHDF), as their molecular size is smaller than the filter pores' cutoff. The study's main aim was to assess whether the serum concentration of thyroid hormones changes over time during CVVHDF. Methods: This was a prospective observational trial that included 30 patients treated in an ICU. All patients developed acute kidney injury (AKI) and had clinical indications for implementation of CVVHDF. Blood samples were collected before initiation of CVVHDF and at 1, 2, 3, 6, 9 and 12 days after. The last sample was collected three days after CVVHDF withdrawal. Thyroid function was evaluated by determining the serum concentration of TSH, thyrotropin-releasing hormone (TRH), free triiodothyronine (fT₃), free thyroxine (fT₄), total triiodothyronine (tT₃), total thyroxine (tT₄) and reverse triiodothyronine (rT₃). We additionally calculated the total activity of peripheral deiodinases (G_D) using a mathematical model. Results: TRH and TSH levels remained mostly within normal ranges. fT4 and tT4 were in normal range or slightly below. In contrast, fT3 and tT3 were undetectably low in most patients throughout. Reverse T3 levels remained within normal limits. There were no statistically significant changes in any thyroid hormone levels over the CVVHDF treatment period. The calculated peripheral G_D activity was lower than normal, but importantly, it did not change significantly over time. Conclusions: Thyroid hormones are not lost due to hemodiafiltration. Decreased deiodinases activity is responsible for alterations in serum concentrations of thyroid hormones in patients during CVVHDF.