Police divers have joined the search
Brendan Rushe was last seen outside a nightclub in Donegal town in the Republic of Ireland in the early hours of Monday morning.
The 15-year-old, from Castlederg, vanished after leaving friends at a disco saying he was going to catch a later bus home.
Police in the Republic of Ireland have said they are keeping an open mind about the case.
Despite driving rain, high winds and freezing temperatures on Friday, search teams continued to comb hillsides and ditches for clues about the missing teenager.
A police diving team has been searching the Eske River which flows into Donegal Bay.
An extra 100 Irish police officers have been drafted in from neighbouring counties.
The gardai have asked local farmers to check out outbuildings and hay barns in case the teenager may have got lost.
On Thursday, the police said a boy knocked at the door of a hotel in Donegal town around the time that the teenager went missing.
Police appealed for the driver of a car to come forward, who may have passed the boy as he later ran along Donegal town quay.
Police officers and more than 100 soldiers from the Parachute Regiment and the Prince of Wales's Own Regiment combed the northern side of the Irish border on Thursday.
Hundreds of people from the boy's home town of Castlederg have also been helping police search bogland in County Donegal.
A garda sub-aqua unit has searched Lough Mourne in Donegal and a river near to where a pair of shoes, his mother said were Brendan's, were discovered.
'Atrocious weather'
However, Irish police said the teenager's friends did not believe the shoes belonged to Brendan.
Superintendent John McFadden has paid tribute to the search volunteers.
"The people of Castlederg have really come out to show massive support for the family.
"They have been searching in absolutely atrocious weather," he said.
Brendan is 5ft 8in tall, of slim build and has dark brown short hair and brown eyes. He has braces on his teeth.
He was wearing a cream shirt, grey trousers and black shoes and had a black jumper with him when he went missing.
Anyone with any information is asked to contact the police at Castlederg on 028 81671202.