Check out this link and click on Depression. Talks about Bi-polar and successful trials of the use of T3 medication.
It is from the national academy of hypothyroidism
http://nahypothyroidism.org/
The article suggest evidence that bi-polar/depressed patients have even further impaired transport at the cellular level in the brain which essentially starves the brain of Thyroid even though there is plenty of thyroid in the blood.
You may have to read the whole website above to get a better/clearer understanding of this specific section. But suffice it to say that thyroid levels can be related to bi-polar/depression in a significant way. It may not be the only thing, but it sure can be a big factor.
Hypothyroidism can cause depression and/or make it worse.
Do you have current thyroid labs, that you could post, so we can see where your levels actually are? Often, even if they fall within the "normal" range, that's not good enough.
Please post labs you have and be sure to include reference ranges, since those vary lab to lab and must come from your own report.