Like primary hypertension, secondary hypertension usually has no specific signs or symptoms, even if your blood pressure has reached dangerously high levels.

If you've been diagnosed with high blood pressure, having any of these signs may mean your condition is secondary hypertension:

• High blood pressure that doesn't respond to blood pressure medications (resistant hypertension)
• Very high blood pressure — systolic blood pressure over 180 millimeters of mercury (mm Hg) or diastolic blood pressure over 120 mm Hg
• High blood pressure that no longer responds to medication that previously controlled your blood pressure
• Sudden-onset high blood pressure before age 30 or after age 55
• No family history of high blood pressure
• No obesity

A number of conditions can cause secondary hypertension. Several kidney diseases may cause secondary hypertension, including:

• Diabetes complications (diabetic nephropathy). Diabetes can damage your kidneys' filtering system, which can lead to high blood pressure.
• Polycystic kidney disease. In this inherited condition, cysts in your kidneys prevent the kidneys from working normally and can raise blood pressure.
• Glomerular disease. Your kidneys filter waste and sodium using microscopic-sized filters called glomeruli that can sometimes become swollen. If the swollen glomeruli can't work normally, you may develop high blood pressure.
• Renovascular hypertension. This type of hypertension is caused by narrowing (stenosis) of one or both arteries leading to your kidneys.

Renovascular hypertension is often caused by the same type of fatty plaques that can damage your coronary arteries (atherosclerosis) or a separate condition in which the muscle and fibrous tissues of the renal artery wall thicken and harden into rings (fibromuscular dysplasia). This condition can cause irreversible kidney damage.

Medical conditions affecting hormone levels may also cause secondary hypertension. These conditions may include:

• Cushing syndrome. In this condition, corticosteroid medications may cause secondary hypertension, or hypertension may be caused by a pituitary tumor or other factors that cause the adrenal glands to produce too much of the hormone cortisol.
• Aldosteronism. In this condition, a tumor in one or both of the adrenal glands, increased growth of normal cells in one or both of the adrenal glands or other factors cause the adrenal glands to release an excessive amount of the hormone aldosterone. This makes your kidneys retain salt and water and lose too much potassium, which raises blood pressure.
• Pheochromocytoma. This rare tumor, usually found in an adrenal gland, increases production of the hormones adrenaline and noradrenaline, which can lead to long-term high blood pressure or short-term spikes in blood pressure.
• Thyroid problems. When the thyroid gland doesn't produce enough thyroid hormone (hypothyroidism) or produces too much thyroid hormone (hyperthyroidism), high blood pressure can result.
• Hyperparathyroidism. The parathyroid glands regulate levels of calcium and phosphorus in your body. If the glands secrete too much parathyroid hormone, the amount of calcium in your blood rises — which triggers a rise in blood pressure.

Other possible causes of secondary hypertension include:

• Coarctation of the aorta. With this defect that's generally present at birth, the body's main artery (aorta) is narrowed (coarctation). This forces the heart to pump harder to get blood through the aorta and to the rest of your body. This, in turn, raises blood pressure — particularly in your arms.
• Sleep apnea. In this condition, often marked by severe snoring, breathing repeatedly stops and starts during sleep, causing you to not get enough oxygen.

Not getting enough oxygen may damage the lining of the blood vessel walls, which may make your blood vessels less effective in regulating your blood pressure. In addition, sleep apnea causes part of the nervous system to be overactive and release certain chemicals that increase blood pressure.

• Obesity. As you gain weight, the amount of blood circulating through your body increases. This puts added pressure on your artery walls, increasing your blood pressure.

Excess weight often is associated with an increase in heart rate and a reduction in the capacity of your blood vessels to transport blood. In addition, fat deposits can release chemicals that raise blood pressure. All of these factors can cause hypertension.

• Pregnancy. Pregnancy can make existing high blood pressure worse, or may cause high blood pressure to develop (pregnancy-induced hypertension or preeclampsia).
• Medications and supplements. Various prescription medications — such as pain relievers, birth control pills, antidepressants and drugs used after organ transplants — can cause or aggravate high blood pressure in some people.

Over-the-counter decongestants and certain herbal supplements, including ginseng, licorice and ephedra (ma-huang), may have the same effect. Many illegal drugs, such as cocaine and methamphetamine, also increase blood pressure.

The greatest risk factor for having secondary hypertension is having a medical condition that can cause high blood pressure, such as kidney, artery, heart or endocrine system problems.

Secondary hypertension can worsen the underlying medical condition you have that's causing your high blood pressure. If you don't receive treatment, secondary hypertension can also be associated with other medical conditions, such as:

• Damage to your arteries. This can result in hardening and thickening of the arteries (atherosclerosis), which can lead to a heart attack, stroke or other complications.
• Aneurysm. Increased blood pressure can cause your blood vessels to weaken and bulge, forming an aneurysm. If an aneurysm ruptures, it can be life-threatening.
• Heart failure. To pump blood against the higher pressure in your vessels, your heart muscle thickens. Eventually, the thickened muscle may have a hard time pumping enough blood to meet your body's needs, which can lead to heart failure.
• Weakened and narrowed blood vessels in your kidneys. This can prevent these organs from functioning normally.
• Thickened, narrowed or torn blood vessels in the eyes. This can result in vision loss.
• Metabolic syndrome. This syndrome is a cluster of disorders of your body's metabolism — including increased waist circumference, high triglycerides, low high-density lipoprotein (HDL) cholesterol (the "good" cholesterol), high blood pressure and high insulin levels.

If you have high blood pressure, you're more likely to have other components of metabolic syndrome. The more components you have, the greater your risk of developing diabetes, heart disease or stroke.

• Trouble with memory or understanding. Uncontrolled high blood pressure also may affect your ability to think, remember and learn. Trouble with memory or understanding concepts is more common in people who have high blood pressure.