Encyclopedia of Health and Medicine

170 The Blood and Lymph
aside from the intrusion of regular blood transfusions,
and normal LIFE EXPECTANCY. Beta forms of
thalassemia tend to be more severe and have a
less optimistic outlook. The secondary HEMOCHRO-
MATOSIS (iron accumulations in the tissues) can
become a significant health factor itself, creating a
therapeutic dilemma.
Risk Factors and Preventive Measures
Because thalassemia is genetic and inherited, the
primary risk factors are family history and presence
of the causative GENE mutations. Doctors
advise GENETIC TESTING and GENETIC COUNSELING for
people who have family history of thalassemia. It
is possible for an individual to carry the gene
defect and show no symptoms of the condition,
which can result in passing the gene defect, and
the disease, to the individual’s children.
See also HEMATOPOIESIS; INHERITANCE PATTERNS;
MUTATION; SICKLE CELL DISEASE.
thoracic duct The largest vessel of the lymphatic
system. The thoracic duct collects LYMPH from the
CISTERNA CHYLI and the left upper body, and drains
into the left subclavian VEIN to deliver lymph to
the bloodstream. About the diameter of a pencil,
the thoracic duct extends from the cisterna chyli
in the central trunk to base of the neck, a distance
of about 16 inches, somewhat paralleling the
AORTA. Like a vein, the thoracic duct has smooth-
MUSCLE walls that rhythmically contract and contains
valves to prevent its contents from
backflowing. Muscular movement, such as occurs
with physical activity or exercise, massages lymph
through the thoracic duct toward the subclavian
vein. Several branches of lymph vessels feed into
the thoracic duct as it courses through the chest,
rejoining to form a single segment that intersects
with the subclavian vein beneath the clavicle (collarbone).
For further discussion of the thoracic duct
within the context of BLOOD and lymph structure
and function please see the overview section “The
Blood and Lymph.”
See also LYMPH NODE; LYMPH VESSELS; RIGHT LYM-
PHATIC DUCT.
thrombocyte
See PLATELET.
thrombocythemia A condition of the BLOOD in
which the body overproduces platelets (also called
thrombocytes), resulting in dysfunctional COAGULA-
TION. Thrombocythemia, also called thrombocytosis,
is a myeloproliferative disorder that can be
primary (an independently occurring disorder, also
called essential or idiopathic thrombocythemia) or
secondary (a consequence of other health conditions
or SPLENECTOMY). Doctors do not know what
causes primary thrombocythemia, which occurs
most commonly in people over age 50.
COMMON CAUSES OF SECONDARY THROMBOCYTHEMIA
INFECTION
INFLAMMATORY BOWEL DISEASE (IBD)
iron deficiency ANEMIA LYMPHOMA
RHEUMATOID ARTHRITIS SARCOIDOSIS
TUBERCULOSIS
Wegener’s granulomatosis
Symptoms and Diagnostic Path
The excess platelets in the blood cause disturbances
of coagulation that often result in these
symptoms, which may be subtle or overt:
• easy bleeding, notably from the mucous membranes,
such as frequent nosebleeds (EPISTAXIS),
or from the gastrointestinal tract
• easy bruising
• clotting (thrombosis)
• SPLENOMEGALY (enlarged SPLEEN)
• HEADACHE or dizziness
• hemorrhage
A blood PLATELET level higher than 500,000
platelets per microliter (mc/L) of blood typically
confirms the diagnosis, though the doctor may
choose to do a BONE MARROW biopsy. Bone marrow
biopsy shows an abundance of megakaryocytes,
the parent cells of platelets, oversize platelets, and
platelet fragments.
Treatment Options and Outlook
Treatment for secondary thrombocythemia targets
the underlying condition, with resolution of the
thrombocythemia after improvement in that condition.
Treatment for primary thrombocythemia
aims to suppress myeloproliferation (bone marrow
cell production activity). Common forms of
myelosuppressive therapy are radioactive phos-