THYROID AND THYMUS: REFRESHING SOME FACTS
Professor Abdulsalam Y. Taha *
Thyroid and thymus are two glands located on both sides of the thoracic outlet. The thyroid gland is located in the neck; however, an enlarged thyroid (goiter) may extend into the thorax (specifically into the anterior mediastinum). When more than half the gland is located below the thoracic inlet, the condition is named retrosternal goiter (RSG). RSG is not uncommon and has been identified since 1749 [1, 2]. Although ectopic thyroid tissue may be found anywhere from the tongue to the diaphragm, the thoracic cavity is considered the most common non-cervical location of an ectopic thyroid. Thyroid tissue has been reported to exist in the thymus gland .
The term thymus is probably derived from the Greek word ‟thumos״ meaning spirit or from the Latin word ‟thyme״ referring to a herb of a similar shape [4, 5]. The thymus is derived from the 3rd pharyngeal pouch , located initially in the neck, but during 8th week of gestation, it descends into its final destination in the anterior mediastinum . The thymus is especially well developed and big-sized in the newborn and the growing child  but after puberty, it undergoes a process of gradual involution and is replaced by fatty tissue . Histologically, the thymus consists of lymphocytes, thymocytes and epithelial stroma  beside argentaffin cells [7–9].
The Greek physician Rufus from Ephesus was the first to mention the thymus as a gland in the first century A.D. However, people required centuries to discover its true function. Initially, it was considered the emotional centre of the body. Later on, the thymus was accepted to have no specific function. In the 16th century, Ambroise Pare (1510-1590), a French surgeon, described it as an ‟ excrescence״ whereas others thought of it as a cushion to prevent damage to the heart or over-expansion of the lungs in the newborn. It wasn’t until the 1970s, when subsets of T cells were precisely identified in the thymus, the role of the thymus as a lymphoid organ became clear .
Argentaffin (Kultschitzky) cells are present in all structures derived from the primitive endoderm including derivatives of the pharyngeal arches, the tracheobronchial tree, the gastrointestinal canal, the pancreatic and the bile ducts . However, the chief locations of these cells are the tracheobronchial tree and the gastro-intestinal tracts. In the thyroid gland, argentaffin cells are named C-cells (the parafollicular cells) which produce the hormone calcitonin. Regardless their location, the tumor produced by these cells is called carcinoid tumor (CT) while C-cells give rise to thyroid medullary carcinoma (TMC) which has morphological and behavioral characteristics similar to carcinoid tumors .
The term carcinoid tumor (argentaffinoma) was introduced by Oberndorfer (1907) to designate a tumor which histologically resembled an undifferentiated carcinoma but behaved in a benign fashion . About 85% of the carcinoid tumors are located in the intestine and 10% in the lung . Rosai and Higa identified thymic carcinoid as a separate entity from thymoma in 1972 [9, 10]. Although thymic carcinoid has a similar ultra-structure to carcinoid tumors of other organs , this very rare tumor has a poor prognosis due to its tendency to local and distant spread .
The thyroid, an important endocrine gland in the body, may be affected by several diseases. Hyperthyroidism is most commonly caused by Grave’s disease while hypothyroidism is most commonly caused by Hashimoto's thyroiditis; both are autoimmune disorders. On the other hand, the thymus, the important lymphoid organ, can be affected by a serious autoimmune condition known as myasthenia gravis (MG). Proptosis (exophthalmos) and lid lag are physical signs produced by hyperthyroidism while ptosis is a feature of MG. From a surgical point of view, median sternotomy is the principal approach for MG, thymoma and other thymic tumors while goiters may need this approach if located retrosternally.
Thyroid and thymus share many common features. Proximity to the thoracic outlet, the presence of argentaffin cells, occurrence of tumors sharing similar morphological and behavioral characteristics namely TMC and TC, involvement by autoimmune diseases and a similar surgical approach are just few to mention. While a lot is known about thyroid, still we know little about its neighbor gland, the thymus. As the Greek philosopher Socrates said: what I know is that I know nothing. This saying seems to fit the thymus gland as the more you deal with it the more you believe you know very little about it .
1. Sheng YR, Chong Xi R. Surgical approach and technique in retrosternal goiter: Case report and review of the literature. Annals of Medicine and Surgery 2016; 5:90-92
2. Coskun A, Yildirim M and Erkan N. Substernal Goiter: When is a Sternotomy Required? Int Surg 2014; 99:419–425. DOI: 10.9738/INTSURG-D-14-00041.1
3. Kesici U, Koral O, Karyağar S et al. Missed retrosternal ectopic thyroid tissue in a patient operated for multinodular goiter. Ulus Cerrahi Derg 2016;32:67-70. DOI: 10.5152/UCD.2015.2916
4. Bushan K, Sharma S, Verma H. A Review of Thymic Tumors. Indian J Surg Oncol 2013; 4(2):112–116 DOI 10.1007/s13193-013-0214-2
5. Anastasiadis K, Ratnatunga C, editors. The Thymus Gland: Diagnosis and Management. Berlin Heidelberg New York: Springer, 2007
6. Faller A, Schuenke M, Schuenke G. The Human Body: An Introduction to Structure and Function. 1st English Ed. Stuttgart, New York: Theme, 2004
7 .Hughes JP, Ancalmo N, Leonard GL, Ochsner JL. Carcinoid tumor of the thymus gland: report of a case. Thorax 1975; 30:470-475
8. Rao U, Takita H. Carcinoid tumor of possible thymic origin: a case report. Thorax 1977; 32:771-776
9. Wang D-Y, Chang D-B, Kuo S-H, Yang P-C, Lee Y-C, Hsu H-C et al. Carcinoid tumors of the thymus. Thorax 1994; 49:357-360
10. Ahn S, Lee JJ, Ha SY, Sung CO, Kim J, Han J. Clinicopathological Analysis of 21 Thymic Neuroendocrine Tumors. The Korean Journal of Pathology 2012; 46: 221-225 http://dx.doi.org/10.4132/KoreanJPathol.2012.46.3.221.
* Professor Abdulsalam Y. Taha
Department of Thoracic and Cardiovascular Surgery /College of Medicine/ University of Sulaimani. Senior Consultant Cardiothoracic and Vascular Surgeon/Sulaimani Teaching Hospital /Sulaimani/ Kurdistan/Iraq