Declan Thomson died in hospital in the early hours of Sunday after becoming unwell at a house in Cumbernauld, North Lanarkshire.
A post-mortem examination is due to be carried out but it is understood his death may be drug-related.
Declan’s mother Linda Ann told the Scottish Sun her son fell ill at a party celebrating his 18th birthday and her loss has come just a year after his older brother Dean died.
A 17-year-old woman has been arrested and was due to appear at Airdrie Sheriff Court earlier this week.
Two boys aged 15 were also arrested and a report has been sent to the procurator fiscal in connection with drugs offences.
A Police Scotland spokeswoman said: “An 18-year-old man was admitted to Monklands and District General Hospital at around 2.30am on Sunday after becoming unwell at a house in Cumbernauld. Sadly, he died within hospital later that day.
“The death is being treated as unexplained, however a post-mortem will be carried out in due course to establish the exact cause of death.
“A report will be submitted to the procurator fiscal.”
An online fundraiser to help with Declan’s funeral costs has collected more than £1,300.
A message on the GoFundMe page created by family friend Kim Lennox says: “I hope everyone can pull together and help a little towards Declan’s funeral as we all know the family have had a hard year losing their middle son Dean just over a year ago and now their youngest son Declan.”