Not in Pathfinder. Usually, the "best" choice is to "finish" with levels in one of the classes (or a prestige class with full casting).
If allowing 3.5 material, there are a few other options, but they tend to have very specific requirements... and are probably better to take first, "finishing" with mystic theurge (rather than the other way around).
There is always cheesy early entry methods if MT doesn't please you.
Equipment Trick Sunrod means you can treat light spells as a level higher, meaning with a single level in a spellcasting class, you can cast 2nd level spells. You could take your first MT level as soon as 4th level.
Racial SLA's are no longer legal.
It could also be argued that taking levels in Evangelist, using Mystic Theurge as the class it increases, could also work, allowing you to take extra levels that effectively increase Mystic Theurge.
I don't think the argument is valid.
There are only 10 levels of mystic theurge, so +1 level in mystic theurge's spellcasting won't do anything beyond level 10.
If you are allowed Cultist of Charun as a clerical archetype, you can use Magehunter's trick to enter MT at level 4, with 3 levels of cultist, and at level 15 take levels of arcane trickster, probably advancing as a Cultist spellcaster. It is not that powerful by that level, but it is possible.