I have been suffering from under-active thyroid since I was 15 because of eating disorders. I wasn't diagnosed until I was 20. I was first diagnosed with depression at the age of 16 and I know that I had it earlier on but didn't know what it was. My thyroid was always borderline hypothyroidism but it started getting worse as I started to get into my early 20's. I was getting more depressed and antidepressants didn't work. My doctor prescribed me levo-thyroxine and it saved me.
My depression was never cured and still isn't. I was worse 3 years ago when PTSD symptoms along with depression started. After getting the blood tests done, it was discovered that my TSH levels were way too high and my hypothyroidism was way above the borderline stage. This time, none of the antidepressants worked until I was re-introduced to levo-thyroxine. After 6 weeks of taking the thyroid med and antidepressants, my depression symptoms started to slow down.
I have done research on this and it has been discovered that once your thyroid is affected it needs to be maintained at the right levels or else you'll suffer from depression and other health issues. In order to help you with your physical (weight gain, cold hands and feet, panic attacks, hair falling, low metabolism, low energy) and mental health, you may need to take medication for thyroid for the rest of your life. I have been taking the med for over 18 months now and I'm a lot better than I was 18 months ago.
Also get your doctor to check your Thyroid antibodies. You may also have Hashimoto's autoimmune disease.
This is from my personal experience.