SummaryAlterations in thyroid indices in critically ill dogs were studied retrospectively to determine the incidence of the euthyroid sick syndrome and to assess its prognostic relevance to survival. Sixty‐seven dogs were classified as euthyroid bases on a TSH stimulation test. Forty‐one of 67 (61%) of the euthyroid dogs had low baseline serum T4concentrations (<1.5 ug/dl) and 38 of 67 (56%) of the euthyroid dogs had low baseline serum T3concentrations (<75 ng/dl). There was no significant difference between either the baseline or the post‐stimulation serum T4concentrations when values of dogs that survived were compared with those that died. There was, however, a significant difference (P<0.05) between both the baseline and the post‐stimulation values for T3 when survivors (100.3± 81 ng/dl, 143± 66 ng/dl) and non‐survivors (64.2 ± 17.1 ng/dl, 96.6 ± 38.3 ng/dl) were compared. The euthyroid sick syndrome occurs commonly in critically ill dogs. Further, the extent of depression of serum concentrations of T3may be correlated