Synonym/Acronym
T4, FT4.
Rationale
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.
| T4 |
|---|
| Age | Conventional Units | SI Units (Conventional Units × 12.9) |
|---|
| Cord blood | 6.6–17.5 mcg/dL | 85–226 nmol/L | | Newborn | 5.4–22.6 mcg/dL | 70–292 nmol/L | | 1 mo–23 mo | 5.4–16.6 mcg/dL | 70–214 nmol/L | | 2–6 yr | 5.3–15 mcg/dL | 68–194 nmol/L | | 7–11 yr | 5.7–14.1 mcg/dL | 74–182 nmol/L | | 12–19 yr | 4.7–14.6 mcg/dL | 61–188 nmol/L | | Adult | 5.5–12.5 mcg/dL | 71–161 nmol/L | | Pregnant female | 5.5–16 mcg/dL | 71–206 nmol/L | | Over 60 yr | 5–10.7 mcg/dL | 64–138 nmol/L |
|
| FT4 |
|---|
| Age | Conventional Units | SI Units (Conventional Units × 12.9) |
|---|
| Newborn | 0.8–2.8 ng/dL | 10–36 pmol/L | | 1–12 mo | 0.8–2 ng/dL | 10–26 pmol/L | | 1–18 yr | 0.8–1.7 ng/dL | 10–22 pmol/L | | Adult–older adult | 0.8–1.5 ng/dL | 10–19 pmol/L | | Pregnancy | 0.7–1.4 ng/dL | 9–18 pmol/L |
|
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).
Study type: Blood collected in a gold-, red-, red/gray-, or green-top [heparin] tube; related body system: Endocrine system.
Thyroxine (T4) is a hormone produced and secreted by the thyroid gland. Most T4 in the serum (99.97%) is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin. The remainder (0.03%) circulates as unbound or free T4, which is the physiologically active form. Levels of free T4 are proportional to levels of total T4. The advantage of measuring free T4 instead of total T4 is that, unlike total T4 measurements, free T4 levels are not affected by fluctuations in TBG levels; as a result, free T4 levels are considered the most accurate indicator of T4 and its thyrometabolic activity. Measurement of free T4 is recommended during treatment for hyperthyroidism until symptoms have abated and levels have decreased into the normal range.
Untreated deficiency of T4 in newborns can result in irreversible, severe intellectual deficits and growth impairment. Neonatal screening for hypothyroidism is mandatory in all 50 states.
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
- Discuss how this test can assist in assessing thyroid gland function.
- Explain that a blood sample is needed for the test.
After the Study: Implementation & Evaluation Potential Nursing Actions
Treatment Considerations
- Thyroid alterations can cause changes in physical appearance, such as goiter and exophthalmos, resulting in body image disturbances. Assess the patient’s perception of physical changes and note the frequency of negative comments related to changed physical state. Assist in the identification of positive coping strategies to address changed physical appearance.
Hyperthyroidism
- Discuss symptoms of hyperthyroidism: tachycardia, arrhythmia, palpitations, tremors, sweating, heat sensitivity, weight loss, anxiety, fatigue, insomnia, thinning nails and hair, frequent bowel movements, tremors, sweating, nervousness, anxiety irritability.
- Discuss the symptoms of thyroid storm: fever, vomiting, diarrhea, abdominal pain, confusion, anxiety, coma. Emphasize that this event is life-threatening and should be promptly treated.
- Symptoms of hyperthyroidism in children include tachycardia, sweating, sleeping problems, increased appetite with weight loss, wide-eyed stare, bulging eyes, loose stool, trembling hands, difficulty concentrating.
- Symptoms of hyperthyroidism in infants include tachycardia, rapid breathing, irritability, failure to thrive, diarrhea, increased appetite without weight gain, bulging eyes, skull bones close early.
- Elevated body temperature can occur due to an emotionally labile event that could precipitate a thyroid storm or crisis.
- Ensure the patient’s immediate environment remains cool.
- Encourage the use of light bedding and lightweight clothing to prevent overheating, increase fluid intake to offset insensible fluid loss, encourage bathing with tepid water for comfort and promotion of cooling.
- Administer prescribed antithyroid therapy.
Hypothyroidism
- Discuss symptoms of hypothyroidism: cold sensitivity, fatigue, weight gain, weakness, constipation, thinning hair, hoarseness, muscle weakness, memory impairment, depression, dry skin, muscular aches, and joint pain.
- Symptoms of hypothyroidism in children and teens include delays in growth, puberty, development of permanent teeth, constipation, weight gain without increased dietary intake, swollen or puffy appearance, and poor mental development.
- Symptoms of hypothyroidism in infants include hoarse crying, large protruding tongue, umbilical hernia, constipation, poor muscle tone, difficulty breathing, sleepiness.
- Inadequate thyroid can be associated with decreased cardiac output.
- Monitor and trend vital signs, thyroid laboratory and diagnostic studies (TSH, T3, T4, radioactive iodine uptake), cardiac status indicators, peripheral pulses, skin color, skin temperature, dry scaly skin, periorbital edema, respiratory rate, oxygen saturation, and breath sounds for fluid overload.
- Administer prescribed thyroid hormone replacement medication.
- Altered thought processes can occur with inadequate thyroid function and associated decreased cardiac output and cerebral perfusion.
- An important goal with hypothyroidism would be to collaborate with the HCP to manage the medical problem associated with decreased cerebral perfusion.
- Interventions would be to provide a modified environment that promotes safety, monitor the ability to provide self-care, assess for fall and injury risk, and ensure implementation of appropriate safety measures (e.g., related to violence, fall risk, self-harm risk); administer prescribed thyroid hormone replacement medication.
Clinical Judgement
- Consider how to overcome negative comments related to body image changes, appearance changes experienced due to the disease process.
Follow-Up and Desired Outcomes
- Demonstrates proficiency in selecting clothing that will assist in remaining cool and prevent overheating.
- Demonstrates proficiency in the self-administration of thyroid or antithyroid medication correctly as prescribed.
- Displays acceptance of changed appearance and refrains from negative self-comments.
- Adheres to recommended medication regime.