Familial testotoxicosis: autosomal dominant condition in which Leydig cells differentiate early resulting in spermatogenesis in absence of pituitary hormonal stimulation (i.e., this is a primary testicular disorder).
Congenital Leydig cell hyperplasia: thought to result from high levels of HCG especially as seen in diabetic moms; usually resolves on its own within first month of life; two types have been described:
Diffuse: requires morphometric quantitation.
Nodular: unencapsulated nodule of Leydig cells - usually does not cause precocious puberty.
Leydig cell tumor: neoplastic Leydig cells secrete testosterone, resulting in maturation of adjacent seminiferous tubules; since this is predominantly a local effect, quite often the opposite testis is normal.
McCune-Albright syndrome: testicular maturation before 9 years of age caused by steroid secretion by the testis, resulting in early maturation of hypothalamic-pituitary-testicular axis.
Other: FSH-secreting pituitary adenomas, hypothyroidism, von Recklinghausen's syndrome, etc.