Type of test Blood

Normal findings

1-3 days: 100-740 ng/dL

 1-11 months: 105-245 ng/dL

1-5 years: 105-270 ng/dL

6-10 years: 95-240 ng/dL

 11-15 years: 80-215 ng/dL

16-20 years: 80-210 ng/dL

20-50 years: 70-205 ng/dL or 1.2-3.4 nmol/L (SI units)

>50 years: 40-180 ng/dL or 0.6-2.8 nmol/L (SI units)

Reversed T3 : 10-24 ng/dL

Test explanation and related physiology

As with the thyroxine (T4 ) test, the serum T3 test is an accurate measure of thyroid function. T3 is less stable than T4 and occurs in minute quantities in the active form. Only about 7% to 10% of thyroid hormone is composed of T3 . Seventy percent of that T3 is bound to proteins (TBG and albumin). Abnormal levels (high or low) of thyroid hormone–binding proteins (primarily albumin and TBG) may cause abnormal T3 concentrations in euthyroid patients. This test is a measurement of total T3 (i.e., the free and the bound T3 ). Generally, when the T3 level is below normal, the patient is in a hypothyroid state.

In severe nonthyroidal diseases, T3 levels are decreased because conversion of T4 to T3 in the liver is diminished. This makes T3 levels less useful in indicating hypothyroid states. Instead, there is peripheral conversion of T4 to T3 . This form of T3 is called reverse T3 (rT3 ). Elevated rT3 levels associated with reduced T3 have been observed in starvation, anorexia nervosa, severe trauma and hemorrhagic shock, hepatic dysfunction, postoperative states, severe infection, and burn patients (i.e., “sick euthyroid” syndrome).

Furthermore, there is considerable overlap between hypothyroid states and normal thyroid function. Because of this, T3 levels are mostly used just to assist in the diagnosis of hyperthyroid states. T3 levels are frequently low in sick or hospitalized euthyroid patients.

An elevated T3 level indicates hyperthyroidism, especially when the T4 is also elevated. There is a rare form of hyperthyroidism called T3 toxicosis, in which the T4 is normal and the T3 is elevated.

Interfering factors

• T3 values are increased in pregnancy.

* Drugs that may cause increased levels include estrogen, methadone, and oral contraceptives.

* Drugs that may cause decreased levels include anabolic steroids, androgens, phenytoin, propranolol, reserpine, and salicylates (high dose).

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

 • Blood tube commonly used: red

• Determine whether the patient is taking any exogenous T3 medication because this will affect test results.

• Withhold drugs that may affect results (with physician’s approval).

 Abnormal findings

- Increased levels

- Graves disease

- Plummer disease

- Toxic thyroid adenoma

- Acute thyroiditis

- Factitious hyperthyroidism

- Struma ovarii

- Pregnancy

- Hepatitis

- Congenital hyperproteinemia

Decreased levels

- Hypothyroidism

- Congenital hypothyroidism

- Thyroid surgical ablation

- Myxedema

- Pituitary insufficiency

- Hypothalamic failure

- Protein malnutrition and other protein-depleted states (e.g., nephrotic syndrome)

- Iodine insufficiency

- Renal failure

- Cushing syndrome

- Cirrhosis

- Liver diseases

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