B757 Captain reported the left thrust lever did not advance normally during autothrust takeoff roll and felt blocked from forward movement when the Captain attempted to advance it. Flight crew returned to departure airport and performed a normal landing.

Narrative

After calling for 'auto throttles' on takeoff; the auto throttle system engaged normally and both thrust levers advanced toward takeoff thrust. I called 'check thrust' and we started the takeoff roll. After several seconds we noticed that the left thrust was lagging slightly behind the right. I assumed that it was only a lagging auto throttle; which is typical on the fleet; and I elected to continue the takeoff roll thinking I could adjust it like normal. As I continued to push the left throttle forward; I now noticed that it felt blocked which caused some confusion. Around this point we received an 'auto throttle' caution message. I looked at our airspeed and saw that we were above 100 kts and elected to continue the takeoff since our situation didn't qualify under any of the briefed reasons to reject in the high-speed regime. I felt pressure on from the left thrust lever like it was trying to retard which I still assumed was related to the auto throttles. I reached up and disengaged the auto throttle switch on the Mode Control Panel to try to alleviate the pressure since we were committed to the takeoff. We recall seeing the left engine being at about 1.44 Engine Pressure Ratio (EPR) while the right engine was set to 1.57 EPR. There was no noticeable asymmetrical thrust; and we had a normal takeoff. Upon climbing out the left thrust lever still felt like it was trying to come back so I just held my hand firmly against it to keep all available thrust. We elected to level off at 5000 and get vectors to return to ZZZ. I transferred controls and radios to the FO and I attempted to find a QRH procedure for a mechanically blocked thrust lever. Unable to find an applicable procedure; and the fact that we were now level with plenty of symmetric thrust; I decided to contact Maintenance (MX) for consultation. I made the mistake of contacting hub Maintenance instead of Maintenance Control; but they advised a return to the field which we agreed with. We also contacted Dispatch via ACARS to advise them of the situation. I briefed the Flight Attendants (FA's) and Passengers (pax); and the FO and I briefed the approach. We proceeded to complete our normal decent prep to return. At this point we decided that [requesting priority] would be the safest and most prudent action because of the uncertainty of thrust in a go-around situation. We made a normal landing in ZZZ and taxied back to our original gate. After talking with MX; the Chief Pilot; and supervisor; the FO and I retreated to debrief. We were released from duty and went home.