The levothyroxine sodium hydrate suppository (L-T₄-suppository) is provided as a hospital preparation for the treatment of hypothyroid patients with dysphagia in Japan because only oral preparations of levothyroxine sodium (L-T₄) are approved for the treatment of hypothyroidism. However, it has been found that serum thyroxine and triiodothyronine levels do not increase as expected with the hospital preparation, requiring a higher dosage of L-T₄ in the L-T₄-suppository than in the oral preparations. In this study, to determine an effective thyroid gland hormone-replacement therapy for patients with dysphagia, the pharmaceutical properties of the L-T₄-suppository were investigated. Suppositories containing 300 µg L-T₄ in a base of Witepsol H-15 and Witepsol E-75 (ratio of 1 : 1) were prepared according to Chiba University Hospital’s protocol. Content uniformity, stability, and suppository release were tested. The L-T₄-suppository had uniform weight and content. The content and release property were stable over 90 d when the L-T₄-suppository was stored at 4°C and protected from light. The release rate of L-T₄ increased as pH increased. However, no L-T₄ was released below pH 7.2. The release rate of L-T₄ decreased as temperature decreased. These findings suggest that the low level of release of L-T₄ in the rectum under physiological conditions may be the cause of the low serum thyroxine and triiodothyronine levels following L-T₄-suppository administration.