https://www.scientificamerican.com/article/is-iron-deficiency-causing-your-hair-loss/

A small amount of hair collecting in the shower drain or hairbrush is not necessarily cause for concern. According to the Academy of Dermatology, it's perfectly normal to lose 100 or so hairs from your head every day.

Gradually thinning hair as you age is also normal, and largely hereditary. But hair loss—especially when it's sudden or at a young age—can also be a sign of certain medical conditions or nutrient deficiencies.