Horner syndrome is characterized by miosis, ptosis, facial anhydrosis and enophtalmus and is caused by a lesion along the oculosymphathetic pathway starting from hypothalamus and ending in the eye. The long course of the oculosympathetic pathway predisposes it to a wide variety of pathologic processes, ranging from harmless vascular headaches to life-threatening conditions such as carotid artery dissection or malignancy. Thyroid neoplasms, benign or malign, are among unusual causes of Horner syndrome and represent 1.3-7.8% of cases. Especially, early diagnosis of benign thyroid neoplasms such as multinodular goiter has particular importance because of its reversibility with appropriate treatment. We report a case of Horner syndrome which has been developed as a complication of intrathoracic multinodular goiter.