I was told by my endocrinologist that depression and thyroid disease are like the chicken and the egg...which came first? One could cause the other...or perhaps vice versa. I have an auto immune thyroid condition..."Hashimotos Syndrome" in which for some unkown reason antibodies attack the thyroid gland as if it is diseased. Caused severe depression, anxiety, delusional "conditions", memory loss, night sweat/chills...and a whole lot of other misery. Of course...I was depressed long before the diagnosis...just not as badly. I'm currently regulating it with a high dose of Synthroid [200] and blood tests every three months. That seems to work alright. I would say a simple blood test will let you know what's happening. Totally regulated! Good luck!
m.b.