“It ain’t what you don’t know that gets you into trouble. It is what you know for sure that just ain’t so.” 

This quote, frequently misattributed to Mark Twain, describes the dangers of believing something false with all your heart. Instead of focusing on treating the real problem, you put all of your efforts into fixing an issue that bears little relevance to your future.

This quote was famously used in the film An Inconvenient Truth to highlight the risks associated with denying climate change. Properly attributed or not, the wisdom is hard to ignore: When you believe in something false, you can suffer adverse effects. This is particularly true if that false belief involves your health.

Mental illness is often difficult to diagnose, particularly because there are few physiological tests to help clinicians make a diagnosis. Diabetes is diagnosed through blood tests, cancer is diagnosed through biopsies and medical imaging, but mental illness is largely diagnosed through checklists of self-reported symptoms.

For this reason, mental illnesses, including depression, are sometimes misdiagnosed. According to a 2012 study, 26 to 45 percent of patients referred for “depression” did not meet diagnostic criteria for a depressive illness. A 2009 meta-analysis discovered that general practitioners can only correctly identify depression in 47.3 percent of cases — and many doctors diagnose depression in people who just don’t have it.

Here are 4 conditions that mimic depression — but aren't:

1. Bipolar disorder 

Like depression, bipolar disorder involves periods of intense lows. During these lows, people with bipolar disorder experience the same symptoms found in depression. They may feel hopeless, worthless, or even suicidal.

Unlike depression, however, people with bipolar disorder also experience high periods, or mania, in which they feel confident, productive, or on top of the world. Sometimes this manic phase is so pleasant that people with the disorder are unable to recognize it as part of their illness, and so they only seek help during their low periods.

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Up to 22 percent of people with bipolar disorder are mistakenly diagnosed with depression. Another study found that people with bipolar disorder experience an average gap of 10 years before they receive the proper diagnosis. However, recognizing the difference between bipolar disorder and depression is vital because the medications used to treat depression can often worsen the symptoms of bipolar.

2. Hypothyroidism

In this condition, the thyroid gland does not release a sufficient amount of hormones. Because these hormones are necessary for the brain and body to function, people with this disorder typically experience fatigue, diminished concentration, and a low mood — all characteristics of depression.

Researchers estimate that as many as 20 million Americans have a form of thyroid disease, but up to 60 percent of them are unaware of their condition. Instead, they erroneously believe that they are easily fatigued, lazy, or, yes, depressed. 

Unlike depression, however, people with hypothyroidism are overly sensitive to cold temperatures and may feel cold all the time. They’re also more likely to experience dry skin, hair loss and a hoarse voice. Hypothyroidism can be properly diagnosed through a simple blood test and treatment requires only one pill a day.

3. Diabetes

People often develop Type 2 diabetes without recognizing it. They may suddenly find themselves losing weight, feeling fatigued, and growing irritable. Because all of these symptoms are also associated with depression, people may fail to recognize that their body is having problems with insulin. One precursor to Type 2 diabetes, insulin resistance, has been significantly linked to depression.

People with diabetes are also at risk for “diabetes distress,” a condition that mimics depression, but is due to the stress of living with a chronic illness. 

“Because depression is measured with scales that are symptom-based and not tied to cause, in many cases these symptoms may actually reflect the distress that people are having about their diabetes and not a clinical diagnosis of depression,” said Lawrence Fisher, Ph.D., ABPP.

4. Chronic fatigue syndrome

Also known as myalgic encephalomyelitis or systemic exertion intolerance disease, this condition is characterized by extreme fatigue that has no apparent cause. Chronic fatigue syndrome also involves difficulty concentrating, muscle pain, and problems with sleep — all symptoms associated with depression as well, which is why people with chronic fatigue syndrome are so frequently misdiagnosed.

A 2008 study found that chronic fatigue syndrome is underdiagnosed in more than 80 percent of the people who have it, with depression being the most common misdiagnosis.

Thankfully, there is at least one clear difference between depression and chronic fatigue syndrome: People with depression are both exhausted and uninterested in their hobbies, while those with chronic fatigue may still want to participate in their interests despite their lack of energy.

Depression is a very real, very serious illness that affects millions of people each year. The majority of individuals who are diagnosed with depression are diagnosed correctly and are able to recover with proper therapy and medication. 

Still, a misdiagnosis is always possible when you consider the conditions that look like depression. If you are being treated for depression but aren’t feeling better, it is entirely possible that you have a disorder that mimics the condition.

Before starting an antidepressant regimen, it never hurts to take a blood test to rule out one of the commonly confused conditions, such as hypothyroidism or diabetes.

Remember, “It ain’t what you don’t know that gets you into trouble. It is what you know for sure that just ain’t so.”