Biotin, also known as vitamin B7 or vitamin H, is a cofactor which aids in the metabolism of fats, carbohydrates, and proteins. Biotin deficiency due to diet is pretty rare, but eating raw egg whites for an extended period of time can deplete the body of biotin. Foods that are high in biotin include egg yolks, nuts, meat, and dairy.
The Linus Pauling Institute website has a great in-depth review of biotin that includes information on deficiency symptoms and the function of this vitamin in the body.
Biotinidase is the enzyme that is involved in the extraction of biotin from food and also in the recycling of free biotin that has been used in another reaction in the body.
Profound biotinidase deficiency is defined as having less than 10% of normal enzyme activity, and partial biotinidase deficiency is defined as having between 10% and 30% of normal enzyme activity.
Note that not everyone with a BTD variant is affected by biotinidase deficiency[ref], but for most, a homozygous polymorphism will result in biotinidase deficiency.
So what happens if you are heterozygous for one of these polymorphisms? There isn’t a lot of concrete information about it, but you should produce less of the biotinidase enzyme and thus get less biotin from food sources. Biotin is a water-soluble vitamin and not thought to be toxic at higher levels, so trying a biotin supplement may be worthwhile.
Check your genetic data for rs13078881 D444H (23andMe v4, v5; AncestryDNA)
The following SNPs are considered pathogenic for biotinidase deficiency for those who are homozygous for the minor allele. (This is not a complete list.)
rs28934601 (23andMe v4, v5; AncestryDNA)
rs13073139 (23andMe v4, v5; AncestryDNA)
rs34885143 (23andMe v4, v5; AncestryDNA)
If you are heterozygous for one of the BTD polymorphisms and have symptoms of biotin deficiency, biotin supplements are readily available. It is a water-soluble vitamin and not known to be toxic. Good food sources of biotin include nutritional yeast, which has over 200 mcg per 100g, and chicken liver which has 170 mcg per 130g. [ref]
One thing to note is that higher doses of supplemental biotin can cause errors on lab tests for a variety of hormones and disease. Here is an article explaining it.