The 26-year-old athlete secured a silver at the England Athletics Senior Championships in Bedford.
Ms Iannetelli, who only recently transitioned from sprinting to the technically demanding event, said: “I am quite shocked and also chuffed with that.
“I did not expect a medal at all, I thought fourth or fifth would be a good results for me.
“When I crossed the line it was one of those ‘did that actually happen to me moments’!”
Under the guidance of coach Howard Moscrop, Teresa has made significant progress.
Her second-place finish at the championships is considered her best performance to date.
Despite her success, she remains focused on improvement.
She said: “I am happy with the time but also annoyed because if it hadn’t been so windy, I’m sure I would have run under 60 seconds.”
In addition to the medal, Ms Iannetelli ran a personal best time of 60.63 seconds, placing her on the fringes of the top 10 in the UK.
She currently holds the number one spot in the South West Rankings.
Ms Iannetelli attributes her strong finish to her background as a sprinter.
She said: “Coming into the home straight I was fifth but I know the last 100metres is my strong point with my background as a sprinter.”
The Swindon Harriers celebrated another success at the championships, with Laila-Jae Belgrave clinching a bronze medal.
Competing in the U20s 100m hurdles, Ms Belgrave ran a personal best of 14.62 seconds in the heats and improved to 14.61 seconds in the final.
In the U20 women’s 3000m, Ella Spencer finished sixth in a season’s best time of 9:58.49, while Matt Woodward clocked 1:55.41 in the heats of the senior men’s 800m.
Simon Byrne ran a season’s best of 3:53.36 to finish second in the 1500m A race at the latest British Milers Club regional meeting at the Exeter Arena.