Synonym/Acronym
T3 and FT3.
Rationale
To assist in evaluating thyroid function primarily related to diagnosing hyperthyroidism and monitoring the effectiveness of therapeutic interventions.
Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.
Normal Findings
Method: Electrochemiluminescent Immunoassay.
| T3 |
|---|
| Age | Conventional Units | SI Units (Conventional Units × 0.0154) |
|---|
| Cord blood | 14–86 ng/dL | 0.22–1.32 nmol/L | | 1–3 days | 100–292 ng/dL | 1.54–4.5 nmol/L | | 4–30 days | 62–243 ng/dL | 0.96–3.74 nmol/L | | 1–12 mo | 105–245 ng/dL | 1.62–3.77 nmol/L | | 1–5 yr | 105–269 ng/dL | 1.62–4.14 nmol/L | | 6–10 yr | 94–241 ng/dL | 1.45–3.71 nmol/L | | 16–20 yr | 80–210 ng/dL | 1.23–3.23 nmol/L | | Adult | 70–204 ng/dL | 1.08–3.14 nmol/L | | Older adult | 40–181 ng/dL | 0.62–2.79 nmol/L | | Pregnant woman (last 4 mo gestation) | 116–247 ng/dL | 1.79–3.8 nmol/L |
|
| FT3 |
|---|
| Age | Conventional Units | SI Units (Conventional Units × 1.54) |
|---|
| 0–3 days | 2–7.9 pg/mL | 3.1–12.2 pmol/L | | 4–30 days | 2–5.2 pg/mL | 3.1–8 pmol/L | | 1–23 mo | 1.6–6.4 pg/mL | 2.5–9.9 pmol/L | | 2–6 yr | 2–6 pg/mL | 3.1–9.2 pmol/L | | 7–17 yr | 2.9–5.1 pg/mL | 4.5–7.8 pmol/L | | Adults and older adults | 2.4–4.2 pg/mL | 3.7–6.5 pmol/L | | Pregnant women (4–9 mo gestation) | 2–3.4 pg/mL | 3.1–5.2 pmol/L |
|
Study type: Blood collected in a gold-, red-, or red/gray-top tube; related body system: Endocrine system.
Unlike the thyroid hormone thyroxine (T4), most T3 is converted enzymatically from T4 in the tissues rather than being produced directly by the thyroid gland (see study titled “Thyroxine, Total and Free”). Approximately one-third of T4 is converted to T3. Most T3 in the serum (99.97%) is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin. The remainder (0.03%) circulates as unbound or free T3, which is the physiologically active form.
Levels of free T3 are proportional to levels of total T3. The advantage of measuring free T3 instead of total T3 is that, unlike total T3 measurements, free T3 levels are not affected by fluctuations in TBG levels. T3 is four to five times more biologically potent than T4. This hormone, along with T4, is responsible for maintaining a euthyroid state. Free T3 measurements are rarely required, but they are indicated in the diagnosis of T3 toxicosis and when certain drugs are being administered that interfere with the conversion of T4 to T3.
Factors That May Alter the Results of the Study
T3
- Drugs and other substances that may increase total T3 levels include amiodarone, amphetamine, clofibrate, fluorouracil, insulin, levothyroxine, methadone, opiates, oral contraceptives, phenothiazine, phenytoin, prostaglandins, and T3.
- Drugs and other substances that may decrease total T3 levels include acetylsalicylic acid, amiodarone, anabolic steroids, asparaginase, carbamazepine, cholestyramine, clomiphene, colestipol, dexamethasone, fenclofenac, furosemide, glucocorticoids, hydrocortisone, isotretinoin, lithium, methimazole, netilmicin, oral contraceptives, penicillamine, phenytoin, potassium iodide, prednisone, propranolol, propylthiouracil, radiographic medium (iobenzamic acid, iopanoic acid, ipodate, sodium ipodate, tyropanoic acid), salicylate, and sulfonylureas.
FT3
- Drugs and other substances that may increase free T3 include acetylsalicylic acid, amiodarone, and levothyroxine.
- Drugs and other substances that may decrease free T3 include amiodarone, methimazole, phenytoin, propranolol, and radiographic medium.
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
- Answer any questions or address any concerns voiced by the patient or family.
Hypothyroidism
- Symptoms of hypothyroidism in adults include 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.
Hyperthyroidism
- Symptoms of hyperthyroidism in adults include tachycardia, arrhythmia, palpitations, tremors, sweating, heat sensitivity, weight loss, anxiety, fatigue, insomnia, thinning nails and hair, frequent bowel movements, tremors, sweating, nervousness, anxiety, irritability.
- 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.
Clinical Judgement
- Consider ways to encourage adherence to therapeutic interventions to improve thyroid health.
Follow-Up and Desired Outcomes
- Understands that depending on the results of this study, additional testing may be performed to monitor disease progression and determine the need for a change in therapy.