Thyroxine, Total and Free


T4, FT4.

T4 is a complementary laboratory test in evaluating thyroid hormone levels, a screening test for newborns to detect thyroid dysfunction, and a tool to evaluate the effectiveness of therapeutic thyroid therapy. FT4 is a reflex test for thyroid function to assist in diagnosing hyperthyroidism and hypothyroidism in the presence of an abnormal TSH level. Also used to monitor the effectiveness of thyroid replacement or suppressant therapy.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings
Method: Electrochemiluminescent Immunoassay.

AgeConventional UnitsSI Units (Conventional Units × 12.9)
Cord blood6.6–17.5 mcg/dL85–226 nmol/L
Newborn5.4–22.6 mcg/dL70–292 nmol/L
1 mo–23 mo5.4–16.6 mcg/dL70–214 nmol/L
2–6 yr5.3–15 mcg/dL68–194 nmol/L
7–11 yr5.7–14.1 mcg/dL74–182 nmol/L
12–19 yr4.7–14.6 mcg/dL61–188 nmol/L
Adult5.5–12.5 mcg/dL71–161 nmol/L
Pregnant female5.5–16 mcg/dL71–206 nmol/L
Over 60 yr5–10.7 mcg/dL64–138 nmol/L
AgeConventional UnitsSI Units (Conventional Units × 12.9)
Newborn0.8–2.8 ng/dL10–36 pmol/L
1–12 mo0.8–2 ng/dL10–26 pmol/L
1–18 yr0.8–1.7 ng/dL10–22 pmol/L
Adult–older adult0.8–1.5 ng/dL10–19 pmol/L
Pregnancy0.7–1.4 ng/dL9–18 pmol/L

Critical Findings and Potential Interventions

  • T4: Hypothyroidism: Less than 2 mcg/dL (SI: Less than 26 nmol/L)
  • T4: Hyperthyroidism: Greater than 20 mcg/dL (Greater than 258 nmol/L).

Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.

Consideration may be given to verification of critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory.

At levels less than 2 mcg/dL (SI: less than 26 nmol/L), the patient is at risk for myxedema coma. Signs and symptoms of severe hypothyroidism include hypothermia, hypotension, bradycardia, hypoventilation, lethargy, and coma. Possible interventions include airway support, hourly monitoring for neurological function and blood pressure, and administration of IV thyroid hormone.

At levels greater than 20 mcg/dL (greater than 258 nmol/L), the patient is at risk for thyroid storm. Signs and symptoms of severe hyperthyroidism include hyperthermia, diaphoresis, vomiting, dehydration, and shock. Possible interventions include supportive treatment for shock, fluid and electrolyte replacement for dehydration, and administration of antithyroid drugs (propylthiouracil and Lugol solution).

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