AIM: To determine the therapeutic efficacy and adverse effects of triiodothyronine (T3) as a supplement to the treatment of bipolar depression (BPD) with a mood stabilizer (MS) (lithium - Li, valproic acid - VPA or carbamazepine - CBZ) and the specific serotonin reuptake inhibitor (SSRI), sertraline; and as a supplement to the treatment of unipolar depression (UPD) with sertraline.

METHOD: A random assignment, double blind, placebo-controlled trial separately evaluating patients with a) BPD who are treated with MS + sertraline + T3 or MS + sertraline + placebo and b) UPD who are treated with sertraline + T3 or sertraline + placebo, for up to 8 weeks in both cases. The design of the trial will permit both the outcome of treatment and the speed of response to be evaluated.

Inclusion Criteria:

1. Diagnosis of Major Depressive Episode (MDE) in the context of Bipolar Disorder I or II (BPD-I or BPD-II) or Major Depressive Disorder (UPD), according to DSM-IV criteria, without psychotic features.
2. Hamilton Depression Scale (HAM-D) total >16 with item 1 (depressed mood) >2.
3. Age 18-70 years, male or female.
4. Competent and willing to give written informed consent.

Exclusion Criteria:

1. No clinical hyper- or hypothyroidism nor other thyroid illness.
2. No neurological or other physical illness that may impact upon the study or limit prescription of the study medications.
3. No lifetime history of substance or alcohol dependence or of abuse in the preceding 12 months.
4. No significant suicidal risk (HAM-D item 3 (suicide) <3).