腺 (せん; sen), a kokuji meaning gland, is part of hanja (it is read as ?; seon).
- Tonsil is 扁桃腺 (???; pyeondo seon)
- Thyroid is 甲狀腺 (???; gapsang seon)
Chinese uses 腺 as well (pronounced as xiàn in Mandarin).
腺 is both an associative and phono-semantic compound character (會意兼形聲字). The components are 肉 (flesh) and 泉 (source of water/liquid), so a gland is a part of a body that secretes liquid. Also, 肉 serves as the semantic hint, and 泉 (せん) serves as the phonetic hint.
Udegawa Genshin was the creator of 腺, and he coined it sometime in the late 1700s or early 1800s.
(This is based on my comment on Kirby Cho’s answer.)