The weight of the substance you are measuring by teaspoon will differ, depending on what it is. For instance, a teaspoon of cookie dough will be different from a teaspoon of salt.
All measures under US standards:
1 oz of melted chocolate = aprox 5.6 teaspoons
1 oz of water = aprox 5.8 teaspoons
1 oz of granulated or brown sugar = aprox 7.1 teaspoons
1 oz of confectioner´s...
One cannot put milligrams into a teaspoon, nor can one put ounces into a teaspoon. Find something you would want to put into your teaspoon, and then ask how many milligrams it weighs.