Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Causes
Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris. : 854  People with darker skin tones are more prone to hyperpigmentation, especially with excess sun exposure.Many forms of hyperpigmentation are caused by an excess production of melanin. Hyperpigmentation can be diffuse or focal, affecting such areas as the face and the back of the hands. Melanin is produced by melanocytes at the lower layer of the epidermis. Melanin is a class of pigment responsible for producing color in the body in places such as the eyes, skin, and hair. The process of melanin synthesis (melanogenesis) starts with the oxidation of L-tyrosine to L-DOPA by the enzyme tyrosine hydroxylase, then to L-Dopaquinone and Dopachrome, which forms melanin.As the body ages, melanocyte distribution becomes less diffuse and its regulation less controlled by the body. UV light stimulates melanocyte activity, and where concentration of the cells is greater, hyperpigmentation occurs. Another form of hyperpigmentation is post inflammatory hyperpigmentation. These are dark and discoloured spots that appear on the skin following acne that has healed. Diseases and conditions
Hyperpigmentation is associated with a number of diseases or conditions, including the following:

Addisons disease and other sources of adrenal insufficiency, in which hormones that stimulate melanin synthesis, such as melanocyte-stimulating hormone (MSH), are frequently elevated. Cushings disease or other excessive adrenocorticotropic hormone (ACTH) production, because MSH production is a byproduct of ACTH synthesis from proopiomelanocortin (POMC).
Undifferentiated connective tissue disease (UCTD) is a disease in which the body mistakenly attacks its own tissues. It is diagnosed when there is evidence of an existing autoimmune condition which does not meet the criteria for any specific autoimmune disease, such as systemic lupus erythematosus or scleroderma. Latent lupus and incomplete lupus are alternative terms that have been used to describe this condition.The term is sometimes used interchangeably with mixed connective tissue disease, an overlap syndrome. However, MCTD is thought by some researchers to be a clinically distinct entity and is strongly associated with the presence of high titers of ribonucleoprotein (RNP) antibodies.It is estimated that up to 25 percent of people with systemic autoimmune disease could be considered to have UCTD. Signs and symptoms
Disease presentation varies widely from patient to patient, as UCTD is by definition nonspecific. Symptoms typically include constitutional complaints that are common to connective tissue diseases such as fatigue, a general sense of feeling unwell, and fever. Other symptoms associated with UCTD include:
dry eyes
dry mouth
hair loss
joint inflammation
joint pain
oral ulcers
positive ANA test
raynauds phenomenon
sun sensitive rashClinical presentation in some people diagnosed with UCTD may show: 
a decrease in white blood cell count in the blood
anemia
abnormal nerve sensations in the extremities
inflammation of the lining of the heart and/or lungs
a decrease in platelet countLung involvement, such as nonspecific interstitial pneumonia, is a possible disease complication. Diagnosis
There is no official diagnostic criteria for UCTD. Diagnostic testing generally aims to determine whether a patient has a definite or undifferentiated connective tissue disease.