th y ro id g la n d
of the
e n d o c rin e g la n d s ,
w hich helps to regulate the rate of all
the body’s internal processes. The thy-
roid is situated in the front of the neck,
just below the
la ry n x
(voicebox). It con-
sists of two lobes, one on each side of
tra c h e a
(windpipe), joined by a
piece of tissue called the isthmus.
Thyroid tissue is composed of follicular
cells, w hich secrete the iodine-containing
hormones thyroxine (T4) and triiodo-
thyronine (T3), and parafollicular cells
(or C cells), w hich secrete the hormone
c a lc it o n in .T
and T3 are important in con-
trolling metabolism. Calcitonin helps to
regulate calcium balance in the body
(See also
t h y ro id h o rm o n e s
D is o rd e rs
o f th e th y ro id g la n d
th y ro id h o rm o n e s
The three hormones produced by the
r o id g la n d
are thyroxine (T4) and triiodo-
(T3), which regulate meta-
bolism, and
c a lc ito n in
which helps to reg-
ulate calcium levels in the body See
C on trol
o f th y ro id h o rm o n e p ro d u c tio n
box, overleaf.
th y ro id itis
Inflammation of the
t h y ro id g la n d
occurs in several different forms. The
most common is
H a sh im o to ’s th y ro id it is
a u t o im m u n e d is o r d e r
that causes
h y p o -
t h y ro id is m
(underactivity of the thyroid
The function of the thyroid gland is
controlled by both the
p it u it a r y g la n d
and the
h y p o t h a la m u s
so thyroid
disorders may be due not only to
defects in the gland itself, but also
to disruption of the hypothalamic-
pituitary hormonal control system.
Disorders of the thyroid gland may
cause overproduction of thyroid
h y p e rt h y ro id is m )
underproduction of these hormones
h y p o th y ro id ism
), or enlargement or
distortion of the gland.
M y x o e d e m a
G ra ves' d is e a s e
H a sh im o to ’s
th y ro id itis
are common disorders.
G oitre
(enlargement of the thyroid
gland) may sometimes occur with
no accompanying abnormality of
thyroid function.
Congenital disorders
In rare cases, the thyroid gland
is absent at birth, producing
c re tin is m
However, congenital thyroid
deficiency more often takes the
form of underdevelopment or
maldevelopment, in which there is
some thyroid tissue but not enough
to secrete normal amounts of
hormones. A heel-prick blood test
is performed on newborns to screen
for hypothyroidism. Sometimes
the thyroid develops in an abnormal
position in the neck, causing, rarely,
difficulty swallowing or breathing.
Rarely, genetic disorder may
impair the thyroid’s ability to make
hormones. The low blood level of
thyroid hormones results in greatly
increased secretion by the pituitary
gland of thyroid-stimulating
hormone (TSH), which, in turn,
causes the thyroid to enlarge.
Thyroid infection is uncommon; it
leads to
th y ro id itis
(inflammation of
the thyroid gland). Viral infection can
cause an extremely painful gland and
temporary hyperthyroidism.
Thyroid tumours may be either
cancerous or noncancerous. Thyroid
a d e n o m a s
are noncancerous tumours
that may secrete thyroid hormone,
sometimes in large enough amounts
to cause hyperthyroidism.
T h yroid
c a n c e rs
are relatively rare. They may
be suspected if a single firm or hard
lump can be felt in the gland.
Autoimmune disorders
Graves’ disease is a form of thyroid
overactivity thought to be due to
the body’s immune system producing
antibodies that attack the thyroid
gland. In Hashimoto’s thyroiditis,
antibodies damage glandular cells.
Deficiency of thyroid hormone,
which may occur for a variety of
reasons, causes a condition known
as myxoedema, in which the skin
becomes dry and thickened and facial
features become coarse. Constipation,
cold intolerance, and fatigue are other
common symptoms.
Hormonal disorders
Hormonal changes during puberty
or pregnancy may cause a degree of
goitre temporarily. Hyperthyroidism
due to excessive production of TSH
by the pituitary gland is rare but it
may result from a
p itu ita ry tu m o u r.
Iodine deficiency
Because iodine is necessary for the
production of thyroid hormone, its
deficiency may lead to goitre, but it is
uncommon in developed countries.
Severe iodine deficiency in children
may cause myxoedema.
Blood samples may be taken for
t h y r o i d - f u n c t i o n t e s t s ,
in which the
levels of thyroid or pituitary
hormones are measured. The
thyroid gland itself may be
imaged by various
t h y r o i d s c a n n i n g
techniques or ultrasound. In some
cases, such as a suspected tumour,
b i o p s y
may be carried out to
obtain a sample of thyroid tissue
for microscopic examination.
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