I think there is a lot in this idea, certainly worth a bit of extra study, considering the amount of clinical depression diagnosed today.
On a side note, I have also found tyrosine to be very helpful in reducing depression and anger.
Yes, tyrosine can help with some cases of depression due to its effects on the thyroid and by acting as a precursor for dopamine and norepinephrine, which all can cause depression if low.
On the other hand it will not work in all cases since there are so many causes of depression other than low dopamine, low norepinephrine and low thyroid. I can also be low serotonin, low adrenal function, low prostaglandin levels, high prolactin levels, nutritional deficiencies, many medications can cause depression, etc.