In Sindarin a final -th [θ] dissimilated to -s when it followed another th (or dh) earlier in the word. The clearest example of this is úthaes where Tolkien specifically noted the dissimilations:

usahtie “inducement to do wrong”; S. úthaeth > dissimil. úthaes {úsaeth} (VT44/30).

Tolkien wrote (and deleted) úsaeth after úthaes, indicating he was unsure which th would dissimilate, but seems to have decided it was the final one. This is consistent with another example having to do with the feminine name of the Sinda:

thindā. Q thinda/sinda. S thenn. ... Masculine singular ending referring to persons or animals was -on, or after n -or: thennor, pl. -yn, -yr. Feminine -eth, after ð, th -es, pl. -ith, -is. thennon, thenneth/s, thennyn. pl. †thinn, thendrim, thennath (PE17/141).

This second quote indicates the dissimilation would also occur after [ð]. This sound change affected the feminine suffix -eth, but not the class plural suffix -ath, as seen in the plural form thennath above, as well as thoronath the class plural of thoron “eagle” (S/243). Thus it seems this sound change was no longer active in Sindarin, and affected only older words, not newer inflections.

Conceptual Development: Words both beginning and ending with th are fairly common in Gnomish, so it seems unlikely this sound change applied to the earliest conceptual stages of the language. Whether it affected Noldorin is unclear; I have yet to find an example one way or the other.

The úthaes example was pointed out to me by Elaran and the thennes example by Fiona Jallings, which prompted me to write this entry.