BH4 - Tetrahydrobiopterin
BH4 is an enzyme that is used to make serotonin, dopamine, thyroid hormones, melanin and to detox ammonia. It is recharged by folate and/or niacin and/or vitamin C. With certain combinations of the MTHFR gene, some people have a limited supply of BH4. Those people can probably be identified as those who have tendencies towards depression, low energy, all-or-nothing focus, hypothyroid (even subclinical), are pale, and may have elevated blood ammonia. These people will do best on small amounts of very high quality protein, and lots of methyl folate.
Tetrabiopterin seems to be a common misspelling, and is generating a bunch of google hits, so I'll include that here, too for those of us who can't keep *that* many syllables in our heads at once. ;)
Reasons to Suspect
New BH4 can be made from the purine GTP, so high purine foods may be helpful.
Also, the body makes ammonia when it processes protein, and the BH4 pool is depleted when it detoxes that ammonia. Limiting dietary protein to the RDA is the most effective way to keep ammonia levels reasonable.