The stock tumbled 21.4% to close the session at $3.30 a share.
"With weaker Versa Lite sales, we are lowering the midpoint of our 2019 revenue guidance by $95 million to $1.455 billion," the company said in its second-quarter earnings report.
Fitbit previously had guided analysts to expect fiscal-year revenue of $1.55 billion.
While unit sales of the devices, which monitor physical activity, are expected to increase, Fitbit said revenue per unit is likely to decline.
Fitbit posted an adjusted loss in the second quarter of 14 cents a share, narrower than a year-earlier loss of 22 cents and the loss of 18 cents expected by analysts.
Shares were sliding nearly 19.1% on Thursday to $3.40.
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Constable owns none of the securities listed in this story.