Webster's New World Medical Dictionary

T-4 count
A test that counts the number of T-4
cells in the blood, for example, to assess the
immune status of a patient with HIV. Of the various
ways to read a T-4 count test, the best indicator of
health may be the absolute T-4 count, the actual
number of T-cells per unit volume of blood. Also
known as T-helper count. See also 
helper/suppres-
sor ratio; T cell.
thalamotomy
A procedure that is performed via
stereotactic surgery and is designed to destroy part of
the thalamus in order to relieve intractable pain,
seizures, or involuntary movements, as in Parkinson’s
disease.
thalamus
A large ovoid mass within the midbrain
(the interbrain, connected to the cerebral hemi-
sphere) that forms part of the walls of the third ven-
tricle in the brain. The thalamus contains a number
of distinct groups of cells, or nuclei, that function as
relay centers for sensory and other impulses
between the body and the brain and between the
cerebellum, the basal ganglia, and the cerebral cor-
tex. Destruction of part of the thalamus is called
thalamotomy.
thalassemia
A group of genetic disorders that
involve underproduction of hemoglobin, the indis-
pensable molecule in red blood cells that transports
oxygen and carbon dioxide. All forms of hemoglobin
are made up of two molecules: heme and globin. The
globin part of hemoglobin is made up of four polypep-
tide chains. In normal adult hemoglobin (Hb A), the
predominant type of hemoglobin after the first year of
life, two of the globin chains are identical to each
other and are called the alpha chains. The other two
chains, which are also identical to each other but are
different from the alpha chains, are called the beta
chains. In fetal hemoglobin (Hb F), the predominant
hemoglobin during fetal development, there are two
alpha chains and two different chains called gamma
chains. In thalassemia, there is a mutation (change)
in one or both of the alpha or beta globin chains.
Depending on which globin chain is affected, the
mutation leads to underproduction or absence of that
globin chain, a deficiency of hemoglobin, and anemia.
The carriers of heterozygous forms of alpha and beta
thalassemia have red cell anomalies that range from
very mild to severe.