The index points to an optimistic economic outlook when above 100 and a pessimistic one when below 100. In April the index rose 3.4% to 84.7 points, marking its third consecutive month of gains.
Consumer confidence dived 13.0% in May to 55.3 points, while construction sector confidence dropped 7.7% to 49.8 points.
The Turkish economy has slid into recession, contracting 3% in the final quarter of last year, with economists in a Reuters poll forecasting a contraction of 2.5 percent in the first quarter of 2019.
A tumbling lira, domestic demand slowdown and tightening financing conditions have driven the downturn in the economy. The lira has lost 37% of its value against the dollar since the end of 2017.
At 0755 GMT, the lira stood at 6.065 against the U.S. currency, barely changed from Monday's close. Ankara has taken steps through the central bank and state lenders to support the lira in the face of a continued sell-off this year.
A decision to annul the Istanbul mayoral election and re-run it on June 23 has added a source of political uncertainty which has further unsettled market sentiment. The main opposition candidate narrowly defeated President Tayyip Erdogan's AK Party (AKP) in the March vote.