Triiodothyronine (T₃), Total

T₃ has three atoms of iodine, compared with four atoms in T₄. T₃ is more active metabolically than T₄, but its effect is shorter. There is much less T₃ than T₄ in the serum, and it is bound less firmly to TBG.

This measurement is a quantitative determination of the total T₃ concentration in the blood and is the test of choice in the diagnosis of T₃ thyrotoxicosis. It is not the same as the T₃ uptake test that measures the unsaturated TBG in serum. It can also be very useful in the diagnosis of hyperthyroidism. T₃ thyrotoxicosis refers to a variant of hyperthyroidism in which a patient with thyrotoxicosis has elevated T₃ values and normal T₄ values. This test is not reliable in diagnosing hypothyroidism.

Adults: 80–200 ng/dL or 1.2–3.1 nmol/L

Adolescents (12–23 years): 82–213 ng/dL or 1.3–3.28 nmol/L

Children (1–14 years): 105–245 ng/dL or 1.6–3.8 nmol/L

Neonates (1–3 days): 96–292 ng/dL or 1.4–4.4 nmol/L

Pregnancy: 116–247 ng/dL or 1.8–3.8 nmol/L

If radioimmunoassay is used, the result is reported as T₃ RIA.

1. Obtain a 5-mL venous blood sample. Serum is needed.

2. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

1. Increased T₃values are associated with the following conditions:

   1. Hyperthyroidism

   2. T₃ thyrotoxicosis (Graves disease)

   3. Daily dosage >25 μg of T₃ (Cytomel [liothyronine])

   4. Acute thyroiditis

   5. TBG elevation from any cause

   6. Daily dosage >300 μg of T₄

   7. Early thyroid failure

   8. Thyrotoxicosis factitia

   9. Iodine deficiency goiter

2. Decreased T₃values are associated with the following conditions:

   1. Hypothyroidism; however, some clinically hypothyroid patients will have normal levels

   2. Starvation and state of nutrition subacute nonthyroid illness

   3. TBG decrease from any cause

Pretest Patient Care

1. Explain test purpose and procedure. T₄ is usually the first test used in the diagnosis of hypothyroidism or hyperthyroidism, along with the TSH.

2. Have patient avoid strenuous exercise.

3. Do not administer radiopaque contrast for 1 week before testing.

4. If patient is on thyroid therapy, discontinue treatment for 1 month before testing to determine baseline values.

5. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Values are increased in pregnancy and with the use of drugs such as estrogens, methadone, and heroin (see Appendix E).

2. Values are decreased with the use of drugs such as anabolic steroids, androgens, large doses of salicylates, phenytoin, and nicotinic acid (see Appendix E).

3. Fasting causes T₃ level to decrease.