I'm having sort-of the same experience here-
last winter my tropical hibiscus kept most of its leaves, but this winter it looks much worse, too, although its leaves haven't turned brown.

I'd probably just hang in there;
as long as there's some green, there's hope.

You might let the soil dry out a little more between watering;
the plant's metabolism is slower in winter, so it isn't using water as quickly.

hope your (& my!) hibiscuses rally this spring!

Sorry you are having this problem. I potted and brought a small one in but have been also putting it outside in this fine weather. It is blooming although the flower is not as large as when fertilized.

It could be just lack of sunshine. Mine in the plant room that are in the sun each day have not lost a leaf. Do give it a shot at coming back in the Spring.

Have you checked for pests? I have one that is doing that and it had some sort of little critters on it. I sprayed it with neem and seems to be doing a little better. It actually put out a small flower the other day. The one right beside it is doing fine just a few yellow leaves.