To examine the efficacy and overall tolerability of the simultaneous initiation of treatment (coinitiation) with triiodothyronine (T3) and a selective serotonin reuptake inhibitor (SSRI) for major depressive disorder (MDD). Sources of date were Medline/Pubmed, EMBASE, the Cochrane database, and program syllabi from major psychiatric meetings held since 1995. The study selection comprised double-blind, randomized clinical trials comparing T3-SSRI coinitiation therapy versus SSRI monotherapy for MDD. Data were extracted with the use of a precoded form. Data from four clinical trials involving a total of 444 patients with MDD were identified and combined using a random effects model. There was no statistically significant difference in terms of remission rates or response rates at week 1, week 2, or at endpoint between the two treatment groups (SSRI+T3 coinitiation therapy vs. SSRI monotherapy). Pooled response and remission rates at endpoint for the SSRI+T3 versus SSRI monotherapy groups were 64.6 versus 58.5% and 46.8 versus 44.8%, respectively. In addition, there was no statistically significant difference in overall rates of premature discontinuation of treatment, or in the rate of premature discontinuation of treatment owing to inefficacy or intolerance between the two treatment groups. Notwithstanding important methodological differences between the studies included in the meta-analysis in terms of patient characteristics and treatment protocols, these results do not support the notion that simultaneous initiation of treatment of MDD with an SSRI and T3 is more effective than SSRI monotherapy. However, given the etiologically diverse and clinically heterogeneous nature of MDD, it is at least plausible that T3-SSRIs coinitiation therapy may be effective for a particular subgroup of patients including patients with atypical depression or patients with a functional polymorphism of the D-1 deiodinase gene. Clearly, further work is needed to help determine whether there are specific MDD populations that can, indeed, benefit from T3-SSRI coinitiation therapy.