BackgroundTo assess 28-day survival in two pilot groups of septic shock patients with euthyroid sick syndrome (ESS) supplemented with triiodothyronine (T3). MethodsA total of 95 septic shock patients with ESS were divided according to values of the thyroid hormones into low T3 and low T3T4 groups. Among 48 patients with low T3, 24 (50%) were randomized to T3 for 4 days and 24 (50%) to placebo. Among 47 patients with low T3T4, 24 (51%) were randomized to T3 for 4 days and 23 (49%) to placebo. The analysis included 28-day survival as the primary outcome and laboratory with hemodynamics as the secondary outcomes. Laboratory data were analyzed on the day of admission (T0), on the first (T1), third (T2) and seventh day (T3) with hemodynamics analyzed for the first four days. ResultsIn the low T3 population, 18 (75%) patients receiving T3 died at day 28 compared with 8 (33.3%) patients receiving placebo (p = 0.004). In the low T3T4 population, 6 (25%) patients receiving T3 died in ICU compared with 12 (52.1%) patients receiving placebo (p = 0.039). Oral T3 treatment increased mean arterial pressure values at day 1, day 3 and day 7 in the low T3T4 population, (p = 0.015, =0.005 and =0.042 respectively), and had no significant effect on these values in the low T3 population. ConclusionT3 supplementation was associated with a low 28-day mortality rate in patients with low T3T4 but with increased mortality in patients with low T3 ESS. These results suggest caution before initiating thyroid supplementation in septic patients. RegistrationClinTrials.gov (NCT05270798).