If you've been diagnosed with hypertension — high blood pressure — you're not alone. According to the Centers for Disease Control and Prevention, high blood pressure affects 1 in 3 adult Americans, but many people don't even know it.

The exact causes of hypertension are puzzling to most physicians, but several factors may play a role including being overweight, smoking, and lack of exercise. If your weight is under control, you don't smoke, you exercise regularly, and you eat a healthy diet, but your blood pressure is still high, what else could be contributing to your problem?

"The fact is that medical experts admit they don't have a clue why 90 percent of people suffer from what is known as 'essential hypertension,' which is persistent and pathological high blood pressure for which no specific cause can be found," says neurosurgeon Dr. Russell Blaylock, author of The Blaylock Wellness Report. "But there are many uncommon causes of hypertension," he tells Newsmax Health.

Check out the following causes of high blood pressure you may not have considered:

BPA: Bisphenol A or BPA is a chemical used in manufacturing plastic bottles and coating the inside of cans. Several studies, including one published in the journal Hypertension found that systolic blood pressure increased in volunteers by 4.5 mm Hg after drinking two canned beverages a day compared to those who drank two beverages from glass bottles. You can reduce the amounts of BPA in your body by buying foods and beverages in glass containers and avoiding plastic packaging and plastic wraps. Look for "BPA-free" labels.

OTC Medications: Many common over-the-counter medications can raise blood pressure, including non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen (Advil), says Dr. Blaylock. A Swiss study found that acetaminophen (Tylenol) can also raise blood pressure. In addition, cold and flu medications that contain decongestants, including pseudoephedrine, phenylephrine, and oxymetazoline, may all be contributing to high blood pressure if you take them on a regular basis.

Licorice: In addition to candy, licorice is used in other products, including tobacco and beverages. Although licorice has many medicinal purposes and has been used for thousands of years to treat heartburn, colic, and other problems, it contains a compound called glycyrrhizinic acid that can raise blood pressure. Glycyrrhizinic acid can also lower potassium levels in the body, which can cause abnormal heart rhythms. If you're taking drugs to treat high blood pressure, such as hydrochlorothiazide and spironolactone, or ACE inhibitors, licorice can reduce their effectiveness. Studies have found that licorice can also affect blood pressure during surgery.

Sleep Apnea: Experts estimate that 22 million Americans suffer from sleep apnea, and 80 percent of cases are undiagnosed. Restless nights with constant interruptions in breathing causes oxygen levels to fall. The brain responds by sending signals to the body to increase oxygen to the heart and brain, which causes blood vessels to tighten and raises your blood pressure. In addition, the constant lack of sleep can also increase the amount of stress hormones, which can also raise blood pressure.  

Caffeine: Many studies have found that caffeine increases blood pressure, but if affects some people more than others. Some scientists believe that caffeine causes a release of adrenaline, which raises blood pressure. Others speculate it could block hormones that widen blood vessels, causing vessels to constrict and increasing blood pressure. In addition, a study published in the American Journal of Hypertension found that caffeine in an occasional cup of coffee could reduce the effects of medications that lower blood pressure.

Thyroid problems: Both hyperthyroidism (too much thyroid hormone) and hypothyroidism (too little thyroid hormone) can cause high blood pressure. One study published in the journal Hypertension found that patients who had hypothyroidism had substantially higher blood pressure than those with normal thyroid levels. 

Hyperparathyroidism, which is an excess of the parathyroid hormone (PTH) made by the parathyroid glands in the neck, can also raise blood pressure. Many studies have found that when hormone levels were normalized, blood pressure returned to normal.

Sugar: A study published in Open Heart found that sugar may be worse for blood pressure than salt. Researchers said that eating sugar increases insulin levels, which spurs the sympathetic nervous system and results in increased blood pressure. One study published in the International Journal of Obesity found that eating a high-sugar diet for only two weeks can significantly raise blood pressure. Another study, this one published in the journal Metabolism, found that a single 24-ounce drink sweetened with fructose was enough to raise blood pressure.

Loneliness: A study from the University of Chicago found that lonely people have blood pressure readings as much as 30 points higher than people who don't feel lonely. The difference is the equivalent, the researchers said, between a normal blood pressure of 120 and one of 150, which is considered hypertension. Study participants included people aged 50 to retirement age, and the blood pressure differences between lonely and non-lonely people were greatest among the oldest adults.