Lyn Mynott, CEO of the charity Thyroid UK, says they hear from women who spend years being told they are depressed and are given antidepressants when what they really need is thyroxine tablets.
In its early stage, dementia, too, gets mistaken for depression, especially among the under-65s, says Sam Gould, the UK's national campaigns manager at the Alzheimer's Society.
"Depression and dementia share many of the same symptoms, such as concentration and memory problems and feeling withdrawn.
"This is one of the reasons why people with dementia can be misdiagnosed."
Irritable bowel that's really coeliac disease
Irritable Bowel Syndrome (IBS) is an umbrella term for gut symptoms such as bloating, chronic diarrhoea or abdominal pain where there is no other cause.
However, they are also symptoms of coeliac disease - an immune reaction to gluten, the protein found in wheat, barley, rye and oats.
If those affected eat gluten, their immune system launches an attack, causing inflammation to the intestine that can hamper normal absorption of nutrients.
Many people live with coeliac disease for years, having being wrongly told they have IBS.
Official guidelines say that anyone with IBS symptoms should be tested for coeliac disease.
A blood test to check for signs of antibodies being produced by the immune system as a result of gluten can be done at the GP surgery.
While IBS can be treated with anti-spasmodics and sometimes cognitive therapies, patients with coeliac disease must avoid gluten.
Menopause that's an overactive thyroid
Irregular periods, night sweats, bloating, weight gain and mood changes are common symptoms of the menopause.
But more rarely, bloating and irregular periods are the signs of ovarian cancer, and women can be misdiagnosed as menopausal when they have the disease.
Other conditions mistaken for the menopause include an overactive thyroid gland and non-Hodgkin lymphoma (cancer of the vessels and glands of the lymphatic system), which both cause night sweats.
Tiredness that's a sign of liver problems
The number of conditions mistaken for tiredness is long: certain cancers, Lyme disease, which is spread by ticks - even liver diseases such as hepatitis C.
This disease is caught by contact with infected blood and the virus uses liver cells to make more virus.
