Using a custom font with bold text in a UILabel

I have an app where I am setting the font for a UILabel programmatically as below

[label setFont:[UIFont fontWithName:fontName size:[fontSize intValue]]];

wherein fontName is a string type variable that hold font name such as "Helvetica" а fontSize будет содержать размер шрифта. У меня это отлично работает. Now I want to make this text "Bold" how can I do that?

boldSystemFontOfSize 

works for system font. How can I achieve the same for user defined fonts ?

Thanks.