The Pastor of the protestant Izmir Resurrection Church in Western Turkey was arrested in October 2016 on claims of links to "armed terrorist organisations", claims he denies.
Brunson remains in prison in Izmir despite US requests for his release.
He wrote to his wife, Norine, saying:
"I am deeply grateful to all who have prayed for me. This trial - time in prison - has stretched me far more than I ever would have imagined. I have been very weak, had many doubts, felt very alone. I know that God's grace is sustaining me, even when I do not feel that grace, and I know that the prayers of God's people are surrounding me and giving strength. One of my big fears has been that I will be forgotten in prison. Thank you for not forgetting! It is a great encouragement to know there are people praying for me - it reminds me that I am not alone, and that I need to stand firm, with my face pointed in God's direction always. Thank you for standing with me in this most difficult time.
Many blessings to you
Andrew"
Norine Brunson
Miles Windsor from Christian charity Middle East Concern, who are following Andrew's plight, said: "He is finding some comfort from God in that situation and he says in this later letter that God's grace is sustaining him."
Describing what it's been like for Brunson being without his family for over a year, Windsor said: "There's been a huge amount of strain on him as a result of things like missing his daughters' wedding, missing a graduation ceremony and such things like that - it's a very difficult and painful time for Andrew."
He said that the prison was cold and that Brunson had lost weight.
Windsor added that the Turkish government want "to seek a swap with the US government, indeed on the 28th Oct last year, the Turkish President Erdogan openly suggested a deal with the United states offering to exchange Andrew for Fethullah Gulen, the Islamic Cleric who is now based in the US".
Windsor was also very clear that Middle East Concern were "absolutely confident" that the claims against Brunson are untrue and that his arrest is part of a wider purge in Turkey.
Middle East Concern say Christians in Turkey request prayer for Andrew that the Lord will:
a. continue to uphold Andrew, strengthen and encourage him
b. watch over Andrew's family, protecting and providing for them
c. give wisdom to the Turkish lawyer and others working for Andrew's release, and that Andrew will be freed soon
Click here to listen to Miles Windsor's interview with Premier's Alex Williams: