Not only that you have to report to the Immigration office every 90 days, you will also have to ask for a Re Entry Permit once you want to leave the country for what ever reason.
If you stay in Thailand on a One Year Visa or if you have an extension for up to one year of your Non Immigrant Visa and you want to leave the country, you have to get a Re Entry Permit first, before you leave Thailand.
Your Visa will Expire
If you leave the country without a re entry permit, your visa- or the extension of it, is automatically expired. At your next entry to Thailand, you will only get the ordinary 30 days permission.
You can get a Re Entry Permit (for multiple entry) at any Immigration office in the country.
The fee for it is 1000 Baht for a single entry or 3800 Baht for multiple entry.
Note that the information on this page is only for foreigners who stay in Thailand on a One Year Visa or on a one year extension. If you come to Thailand for holiday or if you have to make a visa run every now and then, you don’t need a Re Entry Permit.