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A - Z Index

glossary a-z index

T Cell

Also Known As: T Lymphocyte

T LymphocyteAudio (En español)

Also known as: T Cell

A type of lymphocyte. There are two major types of T lymphocytes: CD8 cells (cytotoxic T lymphocytes) and CD4 cells (helper T lymphocytes); both T cell types are essential for a healthy immune system. HIV infects and destroys CD4 cells, gradually destroying the immune system.

See Related Term(s):  CD4 T Lymphocyte, CD8 T Lymphocyte, Lymphocyte

T-Cell Depletion

Also Known As: T-Cell Exhaustion

T-Cell ExhaustionAudio (En español)

Also known as: T-Cell Depletion

The gradual decrease in T-cell function that can occur with chronic infections and cancers. T-cell exhaustion weakens the immune system, making it difficult for the body to fight off infections or kill cancer-causing cells.

T-Tropic Virus

Also Known As: X4-Tropic Virus


Also Known As: TID

TachycardiaAudio (En español)

Abnormal rapid heartbeat. In adults, a rate over 100 beats per minute is usually considered tachycardia. Tachycardia can occur as part of lactic acidosis, which may be caused by advanced HIV infection or some antiretroviral (ARV) drugs.

TachypneaAudio (En español)

Abnormal increased rate of breathing. Tachypnea can occur as part of lactic acidosis, which may be caused by advanced HIV infection or antiretroviral (ARV) drugs.

Tanner StagingAudio (En español)

A scale used to classify the onset and progression of puberty in children and adolescents. The scale describes five stages of physical development on the basis of sex characteristics, such as pubic hair growth, development of genitalia in boys, and development of breasts in girls. Because children mature at different rates, health care providers use Tanner staging (in addition to age) to determine appropriate dosing of drugs to treat HIV infection and opportunistic infections.


Also Known As: Tuberculosis


Also Known As: Therapeutic Drug Monitoring


Also Known As: Toxic Epidermal Necrolysis

TeratogenicAudio (En español)

Pertaining to birth defects and developmental malformations. Some antiretroviral (ARV) drugs should be avoided during pregnancy because they can potentially have teratogenic effects on the developing fetus.

TestosteroneAudio (En español)

A type of sex hormone. Testosterone is necessary for developing and maintaining certain male sex characteristics, and it helps maintain muscle mass and bone density. Testosterone deficiency is common with HIV and may result in a decrease in muscle mass, an increase in body fat, or erectile dysfunction.

Therapeutic Drug Monitoring (TDM) Audio (En español)

Measuring the concentration of a drug in the blood at scheduled intervals. Therapeutic drug monitoring (TDM) is used to determine the dose at which a drug will be most safe and effective. Although TDM is not generally recommended for routine use in HIV treatment, it may be considered in some situations.

See Related Term(s):  Therapeutic Index

Therapeutic HIV VaccineAudio (En español)

Also known as: HIV Therapeutic Vaccine

A vaccine to slow the progression of HIV infection or delay the onset of AIDS. To date, no therapeutic HIV vaccine exists, but research is underway.

See Related Term(s):  Preventive HIV Vaccine, Vaccine

Therapeutic Index (TI) Audio (En español)

A ratio that compares the blood concentration at which a drug becomes toxic and the concentration at which the drug is effective. The larger the therapeutic index (TI), the safer the drug is. If the TI is small (the difference between the two concentrations is very small), the drug must be dosed carefully and the person receiving the drug should be monitored closely for any signs of drug toxicity.

See Related Term(s):  Therapeutic Drug Monitoring

ThrombocytopeniaAudio (En español)

A lower-than-normal number of blood platelets.

See Related Term(s):  Platelet

ThymusAudio (En español)

An organ of the lymph system where T lymphocytes (T cells) develop and mature. The thymus is important for normal immune system development early in life and is at its largest size at puberty. The thymus declines in size and function during adult life, eventually being replaced by fat.

See Related Term(s):  Lymphatic System, T Lymphocyte


Also Known As: Therapeutic Index

TIDAudio (En español)

Also known as: t.i.d., tid

An abbreviation for “three times a day.” The abbreviation is commonly used in drug dosing instructions.


Also Known As: TID

TiterAudio (En español)

A laboratory measurement of the concentration of a substance in a solution. For example, an antibody titer measures the presence and amount of antibodies in the blood.

ToleranceAudio (En español)

The ability to tolerate a drug when given as prescribed. In other words, tolerance means benefiting from the drug without having any adverse effects that would make it impossible to continue taking the drug.

TopicalAudio (En español)

Pertaining to a drug or treatment applied to the outer surface of the body, such as the skin or mucous membranes.


Also Known As: Cryptococcosis

Toxic Epidermal Necrolysis (TEN) Audio (En español)

A severe form of Stevens-Johnson syndrome involving at least 30% of the total body skin area.

See Related Term(s):  Stevens-Johnson Syndrome

ToxicityAudio (En español)

Also known as: Drug Toxicity

The extent to which a drug causes adverse effects. Drug toxicity is one of the factors considered when selecting antiretroviral (ARV) drugs to include in an HIV treatment regimen.

See Related Term(s):  Adverse Event

ToxoplasmosisAudio (En español)

An infection caused by the parasite Toxoplasma gondii. Toxoplasmosis is most commonly transmitted by contact with infected cat feces, eating contaminated meat, or drinking contaminated water. Toxoplasmosis can also be transmitted from an infected mother to her child during pregnancy. Symptoms, if any, include swollen lymph nodes and muscles aches that last for a month or more. In fetuses and people with weakened immune systems, toxoplasmosis can cause severe damage to the brain (Toxoplasma gondii encephalitis), eyes, and other organs. In people with HIV, toxoplasmosis of the brain is an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition, Opportunistic Infection

TranscriptionAudio (En español)

The fourth of seven steps in the HIV life cycle. Transcription occurs after HIV has integrated its viral DNA into the DNA of the host cell. During transcription, the host cell uses the genetic instructions carried in HIV DNA to make new HIV RNA, including HIV messenger RNA (mRNA).

See Related Term(s):  Life Cycle, Messenger RNA

TranslationAudio (En español)

The fifth of seven steps in the HIV life cycle. Translation occurs after the host cell makes new HIV RNA. During translation, the host cell uses the genetic instructions carried in the new HIV RNA, specifically messenger RNA (mRNA), to make HIV proteins.

See Related Term(s):  Life Cycle, Messenger RNA

Transmitted ResistanceAudio (En español)

Also known as: Primary Resistance

When a person becomes infected with a strain of HIV that is already resistant to certain antiretroviral (ARV) drugs.

See Related Term(s):  Acquired Resistance, Drug Resistance

TransplacentalAudio (En español)

Passage through or across the placenta. Transplacental usually refers to the exchange of nutrients, waste products, drugs, infectious organisms, or other substances between the mother and the fetus.

Treatment FailureAudio (En español)

When an antiretroviral (ARV) regimen is unable to control HIV infection. Treatment failure can be clinical failure, immunologic failure, virologic failure, or any combination of the three. Factors that can contribute to treatment failure include drug resistance, drug toxicity, or poor treatment adherence.

See Related Term(s):  Clinical Failure, Immunologic Failure, Virologic Failure

Treatment RegimenAudio (En español)

Also known as: Regimen

A structured treatment plan designed to improve and maintain health. Recommended HIV treatment regimens include a combination of three or more antiretroviral (ARV) drugs from at least two different drug classes.

See Related Term(s):  Drug Class

Treatment Simplification

Also Known As: Regimen Simplification

Treatment-ExperiencedAudio (En español)

When a person with HIV is currently taking or has previously taken antiretroviral (ARV) drugs.

See Related Term(s):  Treatment-Naive

Treatment-NaiveAudio (En español)

When a person with HIV has never taken antiretroviral (ARV) drugs.

See Related Term(s):  Treatment-Experienced

TriglyceridesAudio (En español)

A type of fat in blood and adipose (fat) tissue.

Triple-Class ExperiencedAudio (En español)

When an HIV-infected person has received antiretroviral (ARV) drugs from three drug classes—the nucleoside reverse transcriptase inhibitor (NRTI), non-nucleoside reverse transcriptase inhibitor (NNRTI), and protease inhibitor (PI) drug classes.


Also Known As: Viral Tropism

Trough Concentration

Also Known As: Cmin

Trough Level

Also Known As: Cmin

True NegativeAudio (En español)

A negative test result that correctly indicates that the condition being tested for is not present. For example, a true negative HIV test correctly indicates that a person is not infected with HIV.

True PositiveAudio (En español)

A positive test result that correctly indicates that the condition being tested for is present. For example, a true positive HIV test correctly indicates that a person is infected with HIV.

Tuberculin Skin Test

Also Known As: Tuberculosis Skin Test

Tuberculosis (TB) Audio (En español)

An infection caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis. Tuberculosis (TB), also referred to as Mycobacterium infection, is spread when a person with an active infection (TB disease) coughs, sneezes, speaks, or sings, and then a person nearby breathes in the bacteria. TB usually affects the lungs, but it can also affect other parts of the body, such as the kidneys, spine, and brain. There are two forms of TB: latent TB infection and TB disease. In people with HIV, TB is considered an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition, Tuberculosis Disease, Tuberculosis Skin Test

Tuberculosis DiseaseAudio (En español)

The active form of tuberculosis (TB) infection. During TB disease, the bacteria multiply, become active, and make the person sick. A person with TB disease of the lungs can spread TB to others. TB disease primarily affects the lungs, but it can also affect other parts of the body, such as the kidneys, spine, and brain, and it can be fatal. Symptoms include a bad cough that lasts 3 weeks or longer, chest pain, coughing up blood or sputum, weakness, fatigue, loss of appetite, weight loss, fever, chills, and sweating at night. In people with HIV, TB disease is an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition, Opportunistic Infection

Tuberculosis Skin TestAudio (En español)

Also known as: Purified Protein Derivative Test, Tuberculin Skin Test

A screening test for tuberculosis (TB). Purified protein derivative (PPD) extracted from the bacterium that causes tuberculosis is injected just below the skin (intradermally). After 48 to 72 hours, a health care professional checks the site of injection for a reaction that indicates that the person has been exposed to TB. Following a positive TB skin test, additional tests are necessary to determine whether a person actually has active TB (TB disease). Certain populations, such as children, the elderly, or people with weakened immune systems, may have smaller, delayed, or negative reactions to the TB test even if they are infected with TB.

See Related Term(s):  Tuberculosis