Medial s became th (þ or [θ]) before liquids l and r in both Sindarin and Noldorin, as noted by David Salo (GS/§4.144). There are examples in both Sindarin and Noldorin:

• N. os(t) + lond > N. othlond > N. othlon “paved way” (Ety/LOD).
• N. lhos(t) + lhann > N. Lhothlann “Empty and Wide” (Ety/LAD, LUS), a word that reappeared in Sindarin as S. Lothlann (S/123).
• ᴹ✶la(n)sro-ndo > N. lhathron “hearer, listener, eavesdropper” (Ety/LAS²).
• ᴹ√LUS + -ren > N. lothren “wild, waste” (EtyAC/LUS).
• N. os(t) + rond > N. othrond “underground city, fortress” (Ety/OS, ROD), a word that reappeared in Sindarin (WJ/414).
• ✶cas-raya > S. cathrae “tressure” (VT42/12).

One interesting case is N. Cílu < ᴹ✶χīsislōmē (PE21/32). Here it seems the intervocalic [s] became [h] so that after short vowels vanished before morpheme boundaries the result was the cluster [hsl]. This cluster either inhibited or masked the sound change [sl] > [θl], so that the result was [kīlu] after [h] vanished after vowels.

Conceptual Developments: This sound change dates back to the Gnomish of the 1910s, as pointed out by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.5). A couple of representative examples:

• ᴱ✶mᵇāsḷ > G. bothli “oven” (GL/23).
• G. glass “brilliance, clarity” vs. glathrin “brilliant, lucent” (GL/39).