At what income level do you lose mortgage interest deduction?

Your income level does not influence your mortgage interest deduction. Itemizing lets you deduct mortgage interest on the qualified mortgage. You can only use itemization if it exceeds the standard deduction. You cannot get a mortgage interest deduction with a standard deduction. Only itemized deduction allows you to do that. You can only deduct the interest, not the principal, on your mortgage with a mortgage interest deduction. The mortgage interest deduction is just one of the deductions you can itemize. Other deductibles include charitable donations, medical expenses, etc.

Mortgage insurance is what phases out over $100,000 in adjusted gross income. 

You can deduct interest on any mortgage between $750,000 to $1 million of qualified principal based on when you bought the home (before or after 2017). If your mortgage principal balance is lower than that, your loan servicer will send you a 1098 tax form after the end of the year, which shows how much interest you paid during the year. Itemizing the deductions can allow you to deduct the entire amount. 

However, if your mortgage principal is higher than the eligible amount (more than 750,000 for a home bought after 2017, up to $10 million if you bought it before that), your interest will be prorated for deductions. This means that you can only deduct interest on the first $750,000 of principal.