I have no personal experience with the specific protocol you ask for, but Alzet has at least 50 references for thyroid hormone protocols using minipumps for rat research on their website, so I'm sure you can find ample, published documentation which will answers your question. 

http://www.alzet.com/research_applications/T4.html

Dear Björn,

thanks for your quick reply (and sorry for my delayed one). I have already checked the list you mentioned beforehand, but there is no reference refering to an administration of thyroid hormones for a duration of 4 weeks with pump model 2006. The technical support at Charles River's was also uncertain..

My main concern is about the stability of the hormones in vivo: 

Is it necessary to replace the hormone solution during treatment, or not? 

Best,

Yoshiyuki