K–L

Klinefelter’s syndrome A chromosomal disorder
affecting only males in which there is at least one
extra X CHROMOSOME. The normal chromosomal
configuration for a male is XY; the female configu-
ration is XX. The extra X chromosome in a male,
which doctors commonly designate as 47 XXY,
dilutes the SECONDARY SEXUAL CHARACTERISTICS. Men
who have Klinefelter’s syndrome often do not
produce SPERMand thus are infertile (unable to
cause PREGNANCY).
Klinefelter’s syndrome often does not become
apparent until a boy enters (or fails to enter)
PUBERTY. Secondary sexual characteristics are slow
to develop and may appear effeminate, with small
GENITALIA, enlarged breasts (GYNECOMASTIA), and lit-
tle facial HAIR. LEARNING DISORDERSare also com-
mon in boys who have Klinefelter’s syndrome,
though researchers are uncertain of the reason for
this. Adult men often experience SEXUAL DYSFUNC-
TIONsuch as low LIBIDO, ERECTILE DYSFUNCTION, and
INFERTILITY.
The diagnostic path includes BLOOD tests to
measure the levels of TESTOSTERONE, LUTEINIZING
HORMONE(LH), andFOLLICLE-STIMULATING HORMONE
(FSH). Karyotyping, a representation of the chro-
mosomal configuration of the cells, shows the
extra X chromosome (and in some men, more
than one extra X chromosome). Treatment is
TESTOSTERONEsupplementation to restore to nor-
mal the level of testosterone in the blood circula-
tion. Testosterone supplementation generally
results in increased masculinization (appearance
of secondary sexual characteristics) such as thick-
ened beard growth, deepened voice, enlarged
PENISand TESTICLES, and increased MUSCLEmass and
definition. Treatment is generally lifelong.
See also CHROMOSOMAL DISORDERS; GENETIC DISOR-
DERS; KARYOTYPE; TURNER’S SYNDROME.

letdown reflex The release of milk from the lac-

tiferous glands and ducts to the nipple of the

BREASTto initiate BREASTFEEDING(nursing). The first

sucking motions the infant makes when attaching

to the nipple are rapid and pulling. These motions

stimulate the release of OXYTOCINfrom the PITUITARY

GLAND, which causes the tissues around the ducts

to contract to push the milk to the nipple. The

mother feels this release and the initial flow of

milk as a tingling sensation. The more full of milk

the breasts are the more intense the sensation.

Letdown occurs multiple times during a breast-

feeding session. Other events may also stimulate

the letdown REFLEX, such as the sound of the

infant’s cry. Letdown affects both breasts, often

causing milk to leak from the un-nursed breast.

See also PREGNANCY.

libido The level of sexual desire an individual

feels, also called sex drive. Libido represents a

complex interaction between the mind and the

sex hormones. Low hormonal levels often result

in reduced libido. In men the HORMONEassociated

with libido is TESTOSTERONE; in women both ESTRO-

GENSand testosterone play roles in libido. Numer-

ous medications, serious or chronic illness,

long-term ALCOHOLISM, and substance abuse may

also reduce libido. As well, libido typically slows

with age.

Indications or symptoms of low libido may

include

• lack of interest in sex


• lack of sexual arousal


• inability to reach ORGASM


• ERECTILE DYSFUNCTIONin men


• reduced vaginal lubrication in women

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