Hi, 

These plants can be somewhat sensitive. To narrow it down we can ask some questions: 

Is it near a cold window or a heat vent that is getting either a cold draft or drying heat?

It doesn't like low humidity, so you may need to get a cloche for the winter to raise the humidity level. 

Could it be over or under watering causing the leaf browning? Is the pot well draining or sitting in a saucer with water? Make sure to empty any saucers as you water. 

The can be sensitive to water quality as well, with a build up of minerals from either tap water or fertilizing too much and getting a salt accumulation. Using bottled water can be misleading as sometimes there is sodium or minerals added for taste. Have you repotted it recently? Does the soil need to be refreshed? 

Is it getting enough sunlight with the lower winter sun? You may need to supplement light conditions with a grow lamp. 

You can also check the back of the leaves for mites. If you see them you can try to blast off with water first as has the least potential for damage. If that doesn't seem to work you can use an insecticidal soap or horticultural oil for inside plants. You may want to do a test on a leaf first to see if that Calathea is sensitive to burning before applying to the whole plant. 

Let us know if you discover anything else and have questions. In the meantime you can just trim off the browning leaves as well or any that continue to turn brown. 

Emily