Encyclopedia of Health and Medicine

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 configuration
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 SPERM and 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 little
facial HAIR. LEARNING DISORDERS are also common
in boys who have Klinefelter’s syndrome,
though researchers are uncertain of the reason for
this. Adult men often experience SEXUAL DYSFUNC-
TION such as low LIBIDO, ERECTILE DYSFUNCTION, and
INFERTILITY.
The diagnostic path includes BLOOD tests to
measure the levels of TESTOSTERONE, LUTEINIZING
HORMONE (LH), and FOLLICLE-STIMULATING HORMONE
(FSH). Karyotyping, a representation of the chromosomal
configuration of the cells, shows the
extra X chromosome (and in some men, more
than one extra X chromosome). Treatment is
TESTOSTERONE supplementation to restore to normal
the level of testosterone in the blood circulation.
Testosterone supplementation generally
results in increased masculinization (appearance
of secondary sexual characteristics) such as thickened
beard growth, deepened voice, enlarged
PENIS and TESTICLES, and increased MUSCLE mass and
definition. Treatment is generally lifelong.
See also CHROMOSOMAL DISORDERS; GENETIC DISOR-
DERS; KARYOTYPE; TURNER’S SYNDROME.
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letdown reflex The release of milk from the lactiferous
glands and ducts to the nipple of the
BREAST to 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 OXYTOCIN from 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 breastfeeding
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 HORMONE associated
with libido is TESTOSTERONE; in women both ESTRO-
GENS and testosterone play roles in libido. Numerous
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 DYSFUNCTION in men
• reduced vaginal lubrication in women