Thoracic extension of cervical goitre is usually approached through:
"Virtually all Intrathoracic goiters can be removed via a cervical incision." - Schwartz Sx

A thoracic approach (median sternotomy) may be needed in the following cases
patients with primary mediastinal goiters with no thyroid tissue in the neck.
patient with previous thyroid operations who may have developed parasitic, mediastinal vessels.
patients with invasive thyroid cancers for their complete removal or
to control mediastinal bleeding during the operation.

Also know.

A goiter is said to be mediastinal if at least 50% of thyroid tissue is located intrathoracically.
Mediastinal goiters can be

Primary

constitute only 1% of all mediastinal goiters
	arise from ectopic thyroid tissue.
do not have any connection with the thyroid tissue in the neck.
	derives its blood supply from the mediastinum.

Secondary Mediastinal 

arise from downward extension of cervical thyroid tissue.
derive their blood supply from the thyroid vessels of neck.
Neck
Chest
Combined cervico-thoracic route
Thoracoscopic