Test Name: Free T4, Blood
Specimen Type: Serum
Specimen Required: 3-5 Cc Yellow Top Gel Vial
Day Performed: Daily
Reporting Time: 8-24 Hours
Methodology: Beckman Coulter Access II/ Snibe Maglumi 4000
CLINICAL SIGNIFICANCE:
A higher than normal level of free T4 may be due to conditions that involve an overactive thyroid, including:
- Graves’ disease
- Taking too much thyroid hormone medicine
- Thyroiditis
- Toxic goiter or toxic thyroid nodules
- Some tumors of the testes or ovaries (rare)
- Getting medical imaging tests with contrast dye that contains iodine (rare, and only if there is a problem with the thyroid)
- Eating a lot of foods that contain iodine (very rare, and only if there is a problem with the thyroid)
A lower than normal level of free T4 may be due to:
- Hypothyroidism (including Hashimoto disease and other disorders involving an underactive thyroid)
- Severe acute illness
- Malnutrition or fasting
- Use of certain medicines
Test results may also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in a protein that binds T4.
References:
- Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods.24th ed. Philadelphia, PA: Elsevier; 2022: chap 25.
- Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016: chap 78.