Non-contrast computed tomography showed a large goitre with widespread intra-thoracic extension, abutting the aortic arch (fig 1A) as well as compressing the internal and external jugular veins (fig 1B, C). The tracheal anatomy is grossly distorted, giving it the crescentic shape in the coronal plane (fig 1, arrow). As a result, both the trachea and oesophagus are reorientated antero-posteriorly. The patient’s recto-sigmoid colon measured approximately 8.0 cm and was full of faeces (fig 2, arrow). Thyrotropin concentration was 118 mIU/l with undetectable free tetra-iodothyronine concentration. The thyroid autoantibodies were also absent. The megarectum resolved 6 months after initiation of L-thyroxine.

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Figure 1 Computed tomography (CT) scan showing a large goitre with widespread intrathoracic extension, abutting the aortic arch (A) as well as compressing the internal and external jugular veins (B, C).

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Figure 2 CT scan showing the patient’s recto-sigmoid colon measuring approximately 8.0 cm and full of faeces (arrow).

Myxoedema megarectum is an uncommon manifestation of hypothyroidism and is thought to be due to decreased peristalsis, deposition of hydrophilic glycoprotein, and increased oedema of the bowel wall.1 It has also been postulated that neuropathy may develop in the Auerbach’s and Meissner’s plexuses as well as the larger nerve trunks entering the large bowel.2 Although some peristaltic activity is recoverable with thyroxine replacement therapy, in some cases, atony due to degeneration of the bowel muscle is irreversible.3 With progressive myxoedema, the bowel may eventually cease to function altogether, causing generalised gross dilatation of the bowel wall throughout the intestinal tract including the rectum.

In summary, megarectum is uncommonly associated with myxoedema which can be easily missed in the elderly population. Fortunately, it is readily reversible with thyroxine replacement therapy.