Hi Phil and welcome to GoY, where are you on the planet and how warm is the room you have moved your lemon tree into?

Citrus trees lose their leaves for various reasons which are usually stress related and is caused by the following:

1. Change in temperature.

2. over/under watering.

3. Lack of sunlight.

4. Lack of humidity.

As you have brought your lemon plant indoors it is probably one or more of these that is causing your plant to shed its leaves. However, this is natural, it's their way of letting you know that they are not happy about the environment that they are in and they will eventually grow new leaves.

I grow citrus trees in pots, they spend their time on my patio during the summer months and I bring them into my conservatory for the winter.

My advice is to give them as much light as you can, feed them with a proprietary winter citrus feed and mist the leaves daily. Another thing that I do to create humidity is to stand the pots on a an over sized saucer filled with stones which I keep wet. The moisture evaporates from the stones and provides more humidity.

Hope this helps.

I wish I had know that information before I gave my lemon tree away.
Clementine