Acquired diminution of the size of the thymus associated with wasting as from death and reabsorbtion of cells, diminished cellular proliferation, decreased cellular volume, pressure, ischemia, malnutrition, reduced function or malfunction, or hormonal changes. The phenotype is Experimental Factor Ontology identifier EFO_0009875 (thymus atrophy). Also known as: atrophic thymus, atrophied thymus, thymic atrophy.