Diagnosis and causes. When hives of unknown cause lasts longer than six weeks, certain diagnostic tests are indicated. These are performed by a healthcare professional and include tests that measure blood count, level of thyroid hormone, vitamin D levels, inflammation markers, such as a SED rate, and a C-reactive protein are also measured. Liver function is evaluated. Urinalysis and immunological tests can also be performed. If indicated, tests for parasites will be performed as well. Consequently, your healthcare professional is evaluating you for chronic infections for example, hepatitis and herpes simplex virus, auto immune diseases, such as celiac disease or rheumatoid arthritis, endocrine disorders like hypo or hyperthyroidism, like Hashimoto’s thyroiditis. You’ll also be examined for low vitamin D, which can affect your immune function and cause hives.