THYROID SCANNING
T
CONTROL OF THYROID HORMONE PRODUCTION
The blood levels of the hormones T3 (triiodothyronine) and T4 (thyroxine)
produced by the thyroid must be kept within narrow limits, otherwise
hyperthyroidism or hypothyroidism may result. The control systems below exist
to achieve this balance but certain disorders may interfere with the system.
Raised blood levels of thyroid
hormones
Bloodstream
Ifblood levels ofthe thyroid hormonesT3 and
T4 rise, they decrease the sensitivity ofthe
pituitary to thyrotrophin-releasing hormone
(TRH), secreted by the hypothalamus.
Hypothalamus
Secretes TRH.
Pituitary gland
The pituitary becomes less sensitive to TRH,
so it secretes less thyroid-stimulating
hormone (TSH).
Thyroid gland
In response to lowered TSH stimulation, the
thyroid reduces its production of the
hormonesT3 and T4.
Bloodstream
The blood levels of T3 and T4 thus gradually
fall back to normal.
Reduced blood levels of thyroid
hormones
Bloodstream
Ifblood levels ofthe thyroid hormones T3
and T4 fall, the hypothalamus is stimulated
to produce more TRH.
Hypothalamus
Increases secretion of TRH.
Pituitary gland
In response to stimulation by TRH, the
pituitary increases production ofTSH.
Thyroid gland
In response to increased TSH stimulation,
the thyroid increases its production of T3
and T4.
Bloodstream
The blood levels of T3 and T4 thus gradually
rise back to normal.
gland). Less commonly, it is associated
with a viral infection, or it may occur
temporarily in women after childbirth.
th y ro id s c a n n in g
Techniques, such as
r a d io n u c lid e s c a n n in g
and
u lt ra s o u n d s c a n n in g
,
that are able to
Thyroid scanning
This is a gamma scan (scintigram) of a healthy
thyroid gland. An injected radioactive tracer
substance shows areas of activity within the gland.
give information on the location, anato-
my, and function of the
th y ro id g la n d
and detect tumours.
th y ro id -s tim u la tin g h o rm o n e
A hormone that is produced in the
p it u -
it a ry g la n d
,
w hich is located at the base
of the brain. Thyroid-stimulating hor-
mone (TSH) travels in the bloodstream
to the
th y ro id g la n d s
,
w hich are then
stimulated by TSH to release other hor-
mones (known as T3 and T4) that are
involved in the body’s use of energy.
(See also
th y ro id h o rm o n e s
. )
th y ro to x ic o s is
Overactivity of the thyroid gland, also
called
h y p e rt h y ro id is m
.
th y ro x in e
Represented by the symbol T4, the most
important
th y ro id h o rm o n e
.
It is synthes-
ized as
le v o th y ro x in e
and given orally for
h y p o t h y ro id is m
(an underactive thyroid).
TIA
The abbreviation for
t ra n s ie n t is c h a e m ic
a tta c k
,
a brief interruption of the blood
supply to part of the brain
tia b e n d a z o le
A rarely used
a n th e lm in tic d ru g
used to
treat
w o rm in f e s t a t io n s
,
including
s t ro n g y -
lo id ia s is
.
It has largely been replaced by
newer anthelmintics.
tib ia
Also
called the shin, the inner and
thicker of the two long bones in the
lower leg. The tibia is the supporting
bone of the lower leg. It runs parallel to
the
f ib u la
,
the narrower bone to w hich
it is attached by
lig a m e n t s
.
The upper
end articulates w ith the
f e m u r
(thigh
bone) to form the
k n e e
joint; the lower
end articulates w ith the
ta lu s
to form
part of the
a n k le
joint. On the inside of
the ankle, the tibia is widened and pro-
trudes to form a bony prominence
known as the medial malleolus.
FRACTURE
One of the most commonly fractured
bones, the tibia may break across the
shaft or at the upper end from a blow to
the outside of the leg below the knee.
Prolonged walking or running on hard
ground may cause a
s t r e s s f ra c t u re
.
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