How do I echo a variable with a star in it with added text?

So I am creating a variable and I want to echo it with an addition to the end like so:

I have a file: Filename-08-10-2017.txt

I create a variable:

myvariable=Filename*.txt

When I echo that variable:

echo $myvariable

it outputs Filename-08-10-2017.txt

But I want to change the name to .zip

So I am trying to go:

echo $myvariable.zip and have it output Filename-08-10-2017.txt.zip

however it outputs:

Filename*.txt.zip

How do I go about having it output the way I want?

Thanks.

EDIT:

I kind of figured it out.

I saved a new variable as $($myvariable) which saved the output.