www.alphadictionary.com

It struck me - I may be the last to know - that ‘-th’, seemingly an integral part of some words, is actually a suffix. Obvious in such words as ‘growth’ and ‘truth’, it’s less so in ‘bath’, ‘birth’, ‘death’, ‘math’, ‘oath’, ‘ruth(less)’ (from verbs). And ‘dearth’, ‘death’, ‘depth’, ‘health’, and ‘mirth’ (from adjectives).

The specifics in the foregoing screed courtesy of the OED.

According to Etymonline.com, there are actually two '-th's. Here's the page.

Thanks. Their antecedents are similar. Could they be related?
The uses are not so different.