High prolactin question- can anyone answer please?

Hi,

Can anyone help me with my questions about high prolactin. Firstly, can it fluctuate depending what day of your cycle you are, and is it lower when you have your period? also, what is the difference between the measurements mU/L and mIU/L?

Just to give you some background:
After a blood test in August last year, it was revealed that I had high prolactin. I had a blood test on day 18 of my cycle which the result was 609mU/L and then on day 21 of my cylce which the result was 1160mU/L.

My doc then sent me for another blood test the following week which actually happened on day 5 of my next cycle (last day of AF) and the result came back 437mIU/L which was apparently normal (prolactin under 500 is normal I think).

Something was not quite right, so doc then referred me to a specialist, which I couldn't get in to see til december. Before I went I had another blood test in Nov, again on day 21 and the test came back 739 mU/L.

I've been on bromocriptine for two months and just had another blood test (again I had done on day 21) which came back 697 mU/L. Specialist said that it looks like prolactin is lowering, so he's keeping me on bromocriptine.

But my question is....why did my prolactin drop below 500 on day 5 back in August??? And before i started on bromocriptine, my prolactin levels seem to flactuate. I was also tested for prolactinoma, which MRI came back clear.

Would really appreciate some help if anyone has any answers!

Thanks,
Josey