Keratin is the protein that gives strength to cells in the body. It is found in abundance in the skin, hair and nails. I'm thinking that for some people, it can be used as an indicator to guide which vitamins are in shorter supply and/or whether calcium is regulating well. An underproduction of keratin would result in weak nails and hair, while an overproduction can lead to keratosis pilaris and a 'fuzzy' tongue.
My hunch is that keratin production is turned on by calcium entering the cells, and is limited by the availability of the amino acid cysteine. Cysteine is formed from homocysteine using magnesium and vitamin B6.
I'm thinking an overproduction of keratin indicates a need to lower cellular calcium. If there is low coenzyme A, then vitamin B5 may be very helpful to put the cysteine in more useful places.
An underproduction of keratin might respond to an increase of cysteine or might indicate low calcium issues. I'd start by looking at magnesium and vitamin B6. If those don't help, look at calcium, but that's a lot more complicated.