The probe initially reported by the BBC comes after the Scottish company offered 10 people the chance to find a gold can hidden in a case of its beer.
However, some winners then doubted their can’s worth after discovering they are gold-plated instead of solid gold.
One of the winners has asked the Advertising Standards Authority (ASA) to discover whether the £15,000 was incorrectly advertised by the company.
A BrewDog spokesperson has said the use of the term "solid gold" was a mistake, however, the company stood by its £15,000 claim.
ASA confirmed it was looking into the matter.
One of the winners, Adam Dean, a sales manager from Shrewsbury, Shropshire, told the BBC: "I realised I had won after I treated myself to a can after having mowed the lawn and spotted something glistening away in the case.
"It said on the can 'you've won a £15k 24 carat gold Hazy Jane can'. Once I'd got over the shock I wanted to cover it on my house insurance. I got in touch with the can's makers, Thomas Lyte, who told me it was actually brass with a 24 carat gold plating.
"I had it valued by a jewellery expert. He told me it was only worth £500.
“I legally entered a competition to win a solid gold can but I've not got that. I asked for shares to make it up to £15,000 and Brewdog basically said no, so I called the ASA."
In a statement, BrewDog said the £15,000 value was "reasonable based on multiple factors", including the manufacturing price, metal, and quality of the product.
BrewDog added that the cans were collectible items, with the value "somewhat detached from the cost of materials".
The news comes after BrewDog ex-workers posted a letter on Twitter this month which said a "significant number" of former staff had "suffered mental illness as a result of working at Brewdog".
James Watt, BrewDog CEO, has apologised for the ‘pain caused to team members’ and has said action is being taken to prevent further harm.