I Am On Eltroxin Since November And The Dose Was

I am on eltroxin since November and the dose was increased in November. I had a regular period in December and the start of January but I am now 5 days late ( I've always had a 25 day cycle). I did a pregnancy test which read negative. Can eltroxin affect periods/ pregnancy test. Must come off eltroxin shortly for radioactive iodine treatment so need to be sure I'm not pregnant!!!