Brief Answer:
You must not take Valsartan the day of surgery.
You can take 
Synthroid, on the day of your surgery, with a minimal amount of water.
Detailed Answer:
Hello,
Thank you for using HCM.
Is recommended to stop taking ARBs, such as losartan, valsartan and meds ending with “-sartan,” on the day before surgery unless you are on these drugs for 
heart failure rather than 
high blood pressure. The interruption of "-sartan" drugs before surgery, is necessary because these drugs may drop the blood pressure during an 
anesthetic. 
Meanwhile, previously prescribed medications, as thyroid replacement 
hormone therapy (Synthroid and others), may be taken with a minimal amount of water on the day of your surgery. So, Synthroid shall not be discontinued.
If you will experience high blood pressure before the surgery, the doctors will manage the situation, using iv drugs to reset it within the normal ranges.  
Hope this is helpful.
Let me know if you need more clarifications. Otherwise please close and rate the answer.
Kind Regards, 
Dr. Juarda