

Perhaps the listlessness caused by thyroid deficiency could mimic
depression, or feeling unable to do anything could cause one to get
depressed, but I know of no specific effect on the brain caused by the
thyroid that would cause depression. Note that weight gain is usually
a symptom of both. Simple blood tests would indicate if a thyroid
condition is present.

I don't know if depression would cause a reduction in thyroid output,
but I would tend to doubt it. As far as I know clinical depression is
caused by a chemical imbalance in the brain, and that chemical
imbalance has no direct effect on any other part of the body. A regular
everyday depression IMHO should not cause a chemical imbalance in the
body at all.

The pituitary bases its secretions of Thyroid Stimulating Hormone (TSH)
on the level of circulating Thyroxin (there are two types T3 and T4 -
one is used as a reserve and is changed into the other -active- form in
the liver). The ratio of T3 & T4 can be affected by a number of other
hormones (estrogen, for example). Naturally, changing activity of the
body's cells would cause changes in availabilty of free thyroxin, but
the liver and a healthy thyroid should be able to balance things out in
short order.

Good sources for info on the thyroid are the Merk Manual (a physician's
reference book ) although reading it is enough to get one depressed :-)
and the Encyclopedia Brittanica (should be available in your local
library).

I hope this has been of some help.