Have you ever heard of a Tasmanian having two heads?

Or, if you are a Tasmanian, have you ever been asked, "Where's your scar?"

For centuries, people from Tasmania have been the butt of jokes about having two heads, and one anonymous Curious Hobart questioner wanted to know why.

Stefan Petrow, a professor of history at the University of Tasmania, said the joke has long shaped the way other Australians view Tasmanians.

"It's so strongly ingrained in any joke about Tasmania that it often comes up," Professor Petrow said.

So we looked into where the two-headed joke came from, and if there's any truth to it.

To combat iodine deficiency in the population a supplements program began — but it went too far. ( Supplied: Tasmanian Archives and Heritage Office )

Where did the joke come from?

There is little reference to two-headed Tasmanians in historical records, with internet research generating myriad blog posts and amateur documentaries.

But there are three theories, the first of which alludes to Tasmania's historically isolated community and limited choice of mating partners.

The second comes from World War I, when soldiers from the island state allegedly requested two pillows for their bunks instead of the usual one.

Professor Petrow said that explanation was "as good an explanation as any".

An unidentified woman from Europe with goitre, from iodine deficiency. ( Wikipedia: Public Domain )

"I have done a lot of work over the last eight or nine years on Tasmanian soldiers in World War I and I haven't come across that particular point, but it's not impossible," he said.

"Tasmanians were mixing more directly with Australians from other states during the course of the war, so it's certainly possible, but I'd love to see some hard evidence."

But the most plausible explanation is medical and has its basis way back in the glacial period.

Iodine, goitre and the first female MP

The joke about the second appendage is most likely to have come from the widespread cases of goitre in Tasmania throughout the 19th and 20th centuries.

Goitre is the swelling of the neck due to the enlargement of the thyroid gland.

And what causes an enlargement of the thyroid gland? An iodine deficiency.

Paul AC Richards is a retired professor of nuclear medicine, and worked with patients with thyroid conditions throughout his career.

"Tasmania is mildly iodine deficient, and during the 19th and 20th centuries in particular, there was a tremendous amount of goitre in Tasmania," Professor Richards said.

"Sometimes these goitres were very, very large, and so the joke went around that it was protruding like a second head."

Mr Richards said the goitres would grow as large as footballs.

"It was just taken for granted that you had a goitre," he said.

Tasmanian MP Enid Lyons, first female MP in Australian Parliament, pictured in 1950.

Sometimes those goitres were removed, leaving a scar on the neck where the infamous second head would have been.

Famous Tasmanians have not been immune, with Dame Enid Lyons — the first woman elected to the Australian House of Representatives — undergoing goitre removal surgery before the 1949 federal election.

"That's why she wore scarves and necklaces in all her photos after that," Professor Richards said.

He said goitre was so prevalent in Tasmania that the State Government provided daily potassium iodine tablets to schoolchildren in 1949, but the program was eventually dropped as it wasn't an effective measure during school holidays.

So why is Tasmania iodine deficient?

During the ice age, Tasmania's top soil was leached of iodine, resulting in centuries of low-iodine foods.

There has been a concerted effort, particularly in the latter half of the 20th century, to fix the deficiency, with mixed results.

While there is now a very low level of thyroid conditions in Tasmania, the widespread supplementation of iodine in the 1960s provided too much, causing a spate of thyrotoxicosis — hyperthyroidism caused by excessive hormone production.

Leopold Müller's copper engraving depiction of "goitre and cretinism", 1815. ( Wikipedia: Public Domain )

Prof Richards worked extensively with patients at the height of the thyrotoxicosis problem and said they had now got the iodine balance right, but it was tenuous.

"Iodised salt in bread is now mandatory, and iodine is a key ingredient in the products used to clean milk vats and cows' teats before milking," he said.

"But all the dairy companies have to do is switch to a chlorine-based cleaning product and we've got the same problem all over again."

As for the two-headed jibe, the lack of records means it's not possible to be definitive.

Professor Petrow said it was a joke Tasmanians played into as much as mainlanders.

"I think Australians like to poke fun at us in a very convivial way, so they're not having a go at us, but having fun with us," he said.