It's not as easy as it looks.
True Type Fonts are programs that are run to generate an image of characters they represent. In Windows, a built-in facility converts the underlying (usually) vector outlines into bit-map images for display on screen. This easy inclusion in other document processing software does not translate to vector output in PTC drawings. One might think rendering a vector font to a vector drawing is easy, but duplicating the tweaks that others use in interpreting those programs isn't.
In many cases there are separate programs for normal, bold, italic, and bold italic. Since they are programs, their execution can be modified; some programs create false bold or italics by making on-the-fly rendering changes. Most programs depend on the host OS to do the processing, so they don't have to each reinvent the technology and so the results are uniform.
The better bet is to find the actual bold//italic/bold-italic definitions and use those.
Typically the TTF definitions are of precise outlines that are then filled. There is no 'thickness' as there is in stroked fonts, like 'font' that is included by PTC. For other characteristics, the font author will make sure the font doesn't block up using hints.
See https://en.wikipedia.org/wiki/TrueType scroll to the "Hinting language" for information on programming.
Also, http://www.microsoft.com/typography/default.mspx
Also, http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=iws-chapter08