Adenomatous, Unilateral, Retrosternal Extension of the Thyroid Gland presenting as a Mediastinal Mass

Background: Thyroid-related causes of mediastinal masses include retrosternal goiters and thymic enlargement associated with Graves’ disease. Here, we present a case of significant unilateral retrosternal growth of the thyroid gland, presenting as an incidental mediastinal mass, without any evidence of contralateral disease. Associated subclinical hyperthyroidism presents a therapeutic challenge. Clinical Case: A 68-year old gentleman presented to the emergency department with a non-infective exacerbation of known Chronic Obstructive Pulmonary Disease. CT Pulmonary Angiogram revealed a right sided 5.6cm paratracheal mass, which seemed to originate from the posterior aspect of the right lobe of the thyroid and extended to the subcarinal region. The mass displaced the oesophagus and was close to, but did not compress, the trachea. The left thyroid lobe was normal. Thyroid ultrasound was reported as normal, but was later acknowledged to be suboptimal at visualization of the posterior aspect of the gland. Thyroid scintigraphy confirmed increased radionuclide uptake within the mass. Thyroid function tests showed subclinical thyrotoxicosis [TSH 0.04 (0.4-4.0mIU/mL), FT4 17.4 (10-22pmol/L) and FT3 3.36 (2.89-4.88 pmol/L)]. TSH receptor antibody was negative [< 1.1 (<1.75 IU/L)]. On review of prior imaging from other hospitals, the mass had been present since 1999 and was stable in size for at least the past 7 years. The patient was discharged on carbimazole with a plan to perform interval scanning to monitor size. Discussion: Although technetium uptake can be seen in thymus tissue, the identification on imaging that the mass is contiguous with the thyroid gland leads us to believe this is an adenomatous extension of the thyroid gland. Ectopic thyroid tissue is possible, but the size of the mass suggests some prior period of growth. Thyroid carcinoma seems unlikely given the current stability in size, and there are no compressive symptoms, so there is no clear indication for surgery at present. However, given the subnormal TSH level, there is evidence of autonomy, so treatment is indicated. Radio-iodine treatment may be associated with thyroiditis, with attendant swelling of the gland and risk of compression of vital structures, so treatment with ATDs and regular imaging surveillance was deemed most appropriate in his case.