Rashes and patches on the skin
In general, a rash that does not respond to treatment, and is accompanied by other symptoms such as fever, joint pain and muscle aches could be a sign of an internal problem or infection. A rash may also occur due to an allergy or signal a reaction to a medicine, according to the American Academy of Dermatology (AAD).
A velvety rash on the back of the neck or around the arms, usually with a color slightly darker than the person's normal skin tone, is a sign that the patient may have an increased risk for developing type 2 diabetes, Day said. "When I see that, I warn the patient to get their blood sugar checked, and watch their diet."
Less commonly, the velvety rash called acanthosis nigricans could be a warning sign of cancer in an internal organ, such as the stomach or liver, according to Mayo Clinic.
A purple rash on the lower legs that does not respond to topical medication can be a sign of hepatitis C infection, Day said.
Bronzing of the skin and other discolorations
In people with diabetes, a bronzing of the skin can be a sign of a problem with iron metabolism, Day said. A yellowing of the skin, on the other hand, may signal liver failure, and may occur along with the yellowing of the whites of the eyes, Day said.
A darkening of the skin mostly visible in scars and skin folds, as well as on joints, such as elbows and knees could be a sign of hormonal disease, such as Addison's disease, which affects the adrenal glands, according to the AAD.
People who see new growths on the skin should always have them looked at by a doctor, as they could be skin cancer, and are sometimes also a sign of internal disease or a genetic syndrome, according to the AAD.
For example, in a condition called eruptive exanthemas, yellow bumps on the arms, legs or rear could be a result of high triglyceride levels, signaling uncontrolled diabetes, according to the AAD.
The distribution pattern of acne also can provide clues about the underlying problem. In women, acne that appears mainly along the lower face or jaw line can be a sign of polycystic ovarian syndrome, Day said. The condition often causes other symptoms, such as weight changes, thinning hair and increased hair growth on the face, she said.
Changes in the color or shape of nails can often be a sign of deficiency or organ system issues, Day said.
For example, nail changes that look like fungal infection may actually be a result of psoriasis in the nails, even though the condition typically affects the skin. People who also have joint pain could have a form of arthritis called psoriatic arthritis, Day said.
In addition, liver problems and kidney problems can sometimes cause changes in the color of the nails, Day said.
Changes in skin's hardness and dryness
High blood pressure and kidney problems sometimes result in a thickening of the skin on the shin, Day said. Moreover, very dry, itchy skin could be a sign of hormonal problems, such as an underactive thyroid, she said.
"If you are over 30 or 40, and you've never had eczema as a kid and all of a sudden, your skin is dry and you seem to be getting eczema that could be sign of a hormonal issue like low thyroid" function, Day said.
People with an autoimmune disease called systemic sclerosis may experience a swelling and hardening of the skin. In more severe cases, this could result in the hardening of internal organs, such as the lungs or heart, according to the AAD.
On the other hand, very loose and silky skin is a symptom of a rare connective tissue disease, called acquired cutis laxa, which could signal blood cancers such as lymphoma or multiple myeloma, and could progress to affect internal organs, according to the AAD.