parathyroid vs ductless gland

parathyroid and ductless gland both are nouns.

Word	Noun	Adjective	Verb	Adverb
parathyroid	Yes	No	No	No
ductless gland	Yes	No	No	No

As nouns, ductless gland is a hypernym of parathyroid; that is, ductless gland is a word with a broader meaning than parathyroid:

parathyroid: any one of four endocrine glands situated above or within the thyroid gland
ductless gland: any of the glands of the endocrine system that secrete hormones directly into the bloodstream

Other hypernyms of parathyroid include endocrine, endocrine gland.

parathyroid (noun)	ductless gland (noun)
any one of four endocrine glands situated above or within the thyroid gland	any of the glands of the endocrine system that secrete hormones directly into the bloodstream