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Thayer unit

A unit of quantity of vitamin K, mid-20ᵗʰ century, determined by a biological assay. One Thayer unit is the least amount of vitamin K which produces a clotting time of 10 minutes or less in at least 50% of a group of 10 or more chicks, which have been prepared by feeding them a diet lacking vitamin K and then administering the vitamin K orally for three days, the blood being drawn on the fourth day. According to Dam, it is equivalent to about 30 Dam units or 1.5 Ansbacher units.

S. A. Thayer, R. W. McKee, S. B. Binkley, D. W. MacCorquodale and E. A. Doisy.
Proceedings of the Society for Experimental Biology and Medicine, volume 40, page 478 (1939)

S. A. Thayer, R. W. McKee, S. B. Binkley, D. W. MacCorquodale and E. A. Doisy.
Proceedings of the Society for Experimental Biology and Medicine, volume 41, page 194 (1939)

Henrik Dam.
Fat-Soluble Vitamins.
in Annual Review of Biochemistry, volume 9 (1940), pages 353-382.
Palo Alto (CA): Annual Reviews, 1940.

Gives a good review of the Thayer and other biological-assay based vitamin K units of the time.