A bit of a follow-up: some recent blood work, web searching and reading has caused me to experiment with supplementing with tyrosine.
Recent blood work indicated that my TSH was borderline high but my thyroid hormones were borderline low, suggesting a deficiency in the precursors (tyrosine or iodine). I had been running cold, low energy, etc. and have a family history of thyroid issues so I had those levels checked. Also, several years ago I was diagnosed with seasonal affective disorder and was put on a course of Wellbutrin (buproprion) which is a dopamine-reuptake inhibitor. I responded well to it, but after a time I decided that I did not want to be on meds and discontinued their use. Supplementing with vitamin D helped somewhat during the winter, but I read that tyrosine is a precursor to dopamine. It seemed that tyrosine was a common factor in all these issues, so I decided to try supplementing with it.
I've been on tyrosine for two weeks and must say it has brought about a night-and-day difference in mood, even while in deep ketosis and exercising hard in the middle of winter. I'm warm now, energy and enthusiasm are both good, and I am finally seeing exercise as a joy instead of a chore.
I'm continuing on the supplements for now, but tyrosine is an amino acid found in meats, eggs, dairy, nuts and wheat, so in theory I should be able to get a sufficiency through food. I think the problem may have been that I was restricting too much and pushing too hard while not eating enough meat and eggs to fill a chronic tyrosine deficiency. I think I will remain on the supplements while I am still trying to lose weight and then try to phase them out when I reach maintenance.
Tyrosine is apparently something that has been known about by bodybuilders trying to cut: