Engram, 33, has not played in the last nine games.
The return of the 11-year veteran, long regarded as one of the NFL's top slot receivers, comes at an opportune time for the Seahawks, who will be without starting wide receiver Darrell Jackson for the Thursday night contest. Tied with St. Louis' Torry Holt for the league lead in touchdown receptions with 10, Jackson will sit out with a toe injury.
It has been speculated that Jackson, who played in just six games in 2005 because of a knee injury, might be sidelined for several weeks.
No one knows yet how much Engram will play in his return engagement, but the Seahawks will likely monitor his conditioning very closely. Engram lost about 15 pounds while battling the thyroid problem and, despite working out with the scout team the past few weeks, is not yet 100 percent recovered.
The former Penn State standout seems to be past the fatigue that accompanied the thyroid condition and said this week his stamina has improved, but he is still taking medication to control his heart rate.
"We'll take whatever we can get from him," quarterback Matt Hasselbeck said.
The thyroid condition flared up after the Oct. 1 loss at Chicago, when Engram complained of a lack of energy. A battery of tests revealed the thyroid problem. Seattle officials wrestled with the possibility of placing Engram on injured reserve, which would have ended his season, but gambled that he would be able to return late in the year.
Because of injuries, Engram was used more as a starter the past two seasons, but he is best known around the league as a No. 3 receiver and intermediate threat. He has appeared in 139 games and started in 87 of them in stints with the Bears (1996-2000) and Seahawks (2001-06) and has 498 career receptions for 5,962 yards and 29 touchdowns.
Engram has five seasons in which he had more than 50 catches. In four games this season, he has 18 receptions for 198 yards and one touchdown.
Len Pasquarelli is a senior NFL writer for ESPN.com.