**Abstract**

The thymus, a retrosternal lymphoid tissue, develops from the third and fourth pharyngeal pouches like the parathyroid glands at the sixth week of gestational age. The thymus is usually located in the anterior mediastinum, although it can be found anywhere in the thymopharyngeal path. The thymus has a bilobed or quadrilateral shape; however, it can be found in other shapes. Limited information is available about the precise epidemiology of thymic congenital anomalies. Since these anomalies are not symptomatic, it may be more common than the available reports. There are various reports available about the prevalence of thymic diseases and anomalies ranging from 4.45 to 30%. In this chapter we tried to have a review on epidemiology, definition, and management of congenital anomalies of the thymus.

**Keywords:** thymic gland, congenital anomalies, epidemiology, treatments

### **1. History**

The word "thymus" is originally derived from the Greek word "thumos" which means "soul." Previously, in the ancient world, it was believed that the soul is located in this organ. The function of the thymus was a mystery for centuries. The first who defined a role for the thymus was Galen in the second century AD. He believed that the thymus has the responsibility for purification of the nervous system [1, 2]. Later in the fifteenth century, Vesalius mentioned that the thymus works as a shield for protection of major vessels located behind the sternum [2]. The thymus was known as the regulator of fetal and neonatal pulmonary function [1, 2]. For the first time, the thymus was identified as a lymph modifying gland in 1777 by William Hewson. Dissection of corpses let Sir Astley Cooper to write an entire book about the anatomy of the thymus gland [3]. Hassall and Vanarsdale made a remarkable progress in our knowledge about the thymus in 1846 describing the differences between other lymphoid organs and the thymus [4]. Back at the beginning of the twentieth century, finding a relatively large mass (thymus) in autopsy of children who died because of diphtheria was not a fortune. Physicians believed that a smaller thymus presents a normal one [5]. So, some of them used to prescribe radiation therapy for reducing the size of the thymus which resulted in thyroid adenocarcinoma in most of the cases [6]. In 1961, Miller showed the true function of the thymus by describing its destructing effect on the immune system [2, 3].
