Shapechanging into a Hydra

if you shape change into a hydra, does taking 25 damage in your temp HP trigger its trait for head loss and regrowth?

Quote from Shadowmonk88 >>

if you shape change into a hydra, does taking 25 damage in your temp HP trigger its trait for head loss and regrowth?

Yes. It's triggered by taking damage, not by losing HP, so taking damage that only removes temp HP still qualifies.

There's a SAC entry related to TPH and taking damage:

If I have 10 Temporary Hit Points and I take 30 damage from an attack while concentrating on a spell, what is the DC of the Constitution save to maintain my Concentration?

The DC is 15 in that case. When Temporary Hit Points absorb damage for you, you’re still taking damage, just not to your real Hit Points.

In contrast, a feature like the Abjurer Wizard’s Arcane Ward can take damage for you, potentially eliminating the need to make a Constitution saving throw or, at least, lowering the DC of that save.

Also, it's not RAW, but every DM I've done this with has house-ruled that you regain 20 Temp HP, instead of 20 HP, and this temp HP can stack with the temp HP that you currently have when you regrow heads. All of them felt that it just made sense to treat it this way.