No, there's no dh or th or anything like that in Finnish. So dh I think it's just softer version of th.I'll just have to guess here because I don't know. And I'm not sure if I know all the variations of th either, because I've been studying English for about 8 years in shcool and spoken it or used it about the last five years.

_________________
What if?

Yes you're correct, -dh- is the soft, or "voiced" version of th [though there's no -dh- in English in Roman letters of course]. The unvoiced appears in "thin", and for example we have the compound outhouse, in which we have t plus h, not -th-.

Orthography for English is a bit crazy, unlike Finnish or Greek, for examples. With -th-, English speakers simply have to know/learn which word has what sound!

Although if we wrote Maedhros using the Elvish letters, the correct sound would be distinguished by the letter used.