REPRODUCTIVE HORMONES IN THOROUGHBRED STALLIONS:

MONTH	SCROTAL WIDTH (cm)	TESTOSTERONE (pg/mL)	TOTAL ESTROGENS (pg/mL)	FSH (ng/mL)	LH (ng/mL)	TOTAL T4 (ng/mL)	INSULIN (µI.U./mL)
JANUARY	109	1243	330	8.8	1.5	10	2
FEBRUARY	113	1794	309	8.3	1.8	11	2
MARCH	118	1660	391	9.1	2.1	10	2
APRIL	114	1769	308	8.1	4.8	12	2
MAY	112	1627	317	8.3	5.2	8	2
JUNE	116	1306	258	8.1	6.5	8	2
JULY	117	1056	242	7.5	8.0	8	2
AUGUST	118	956	218	7.4	7.0	9	2
SEPTEMBER	107	1155	192	6.3	6.0	8	2
OCTOBER	109	659	203	5.9	2.3	10	2
NOVEMBER	104	515	253	7.4	3.1	10	2
DECEMBER	113	950	269	8.0	3.5	12	2

A. Total Estrogens levels less than 125 pg/mL are abnormal, at any time of the year. The most advanced cases of testicular degeneration have levels less than 100 pg/mL concomitant with levels of FSH exceeding 25 ng/mL.

B. High levels of LH are also present in many cases of testicular degeneration.

C. Insulin levels exceeding 80 µI.U./mL are present in cases of pituitary adenomas. Cushing’s screen would be suggested here, especially if horse is aged.

D. Low Testosterone levels (<500 pg/mL) and/or low Total Estrogens levels (<150 pg/mL) concomitant with normal FSH and LH levels are often present in stallions with low libido.