TSH with Reflex to Free T4

36127

For patients 1 year of age or older, TSH will be performed first.
If it is abnormal (elevated or decreased) based on age/gender specific reference range, then a Free T4 will be performed at an additional charge (CPT code(s): 84439).

1 mL serum

0.7 mL

Specimen collection after fluorescein dye angiography should be delayed for at least 3 days. For patients on hemodialysis, specimen collection should be delayed for 2 weeks.
According to the assay manufacturer Siemens: "Samples containing fluorescein can produce falsely depressed values when tested with the Advia Centaur TSH3 Ultra assay."

Room temperature

Room temperature: 7 days
Refrigerated: 7 days
Frozen: 28 days

Plasma

Set up: Daily; Report available: 1-2 days

Premature Infants (28-36 Weeks)
1st Week of Life	0.20-27.90 mIU/L
Term Infants (>37 Weeks)
Serum or Cord Blood	1.00-39.00 mIU/L
1-2 Days	3.20-34.60 mIU/L
3-4 Days	0.70-15.40 mIU/L
5 Days-4 Weeks	1.70-9.10 mIU/L
1-11 Months	0.80-8.20 mIU/L
1-19 Years	0.50-4.30 mIU/L
≥20 Years	0.40-4.50 mIU/L
Pregnancy
First Trimester	0.26-2.66 mIU/L
Second Trimester	0.55-2.73 mIU/L
Third Trimester	0.43-2.91 mIU/L

TSH levels decline rapidly during the first week of life in most children, but may remain transiently elevated in a few individuals despite normal free T4 levels. For proper interpretation of an abnormal TSH from a newborn thyroid screen, a Free T4 by Dialysis (test code 35167) or Total T4 (test code 867) should be considered.

This test may be useful in assessing thyroid dysfunction when pituitary disease is not suspected. In patients with clinical suspicion of hyperthyroidism or hypothyroidism, testing thyroid stimulating hormone (TSH) is the initial step [1]. An abnormal TSH result will reflex to a free thyroxine (T4) test to aid in diagnosis and guide further testing if needed.

TSH stimulates the thyroid gland to synthesize and secrete triiodothyronine (T3) and T4. TSH production is reduced in response to high T3/T4 levels and increased in response to low T3/T4 levels. When pituitary disease is not suspected, TSH serves as a sensitive marker for screening for thyroid dysfunction [1,2]. A normal TSH result excludes most cases of primary overt thyroid disease. When the TSH level is elevated, measurement of free T4 level may help diagnose subclinical or overt hypothyroidism. Thyroid peroxidase antibody testing may be needed to aid in the diagnosis of Hashimoto thyroiditis. When the TSH level is decreased, measurement of free T4 and free T3 may help identify hyperthyroidism or T3 thyrotoxicosis. In patients with thyrotoxicosis, TSH receptor antibodies testing helps confirm Graves disease [1-3].

Note: Interference due to heterophile antibodies has been known to occur [1].

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References
1. Demers LM, et al. The thyroid: pathophysiology and thyroid function testing. In: Burtis CA, et al. eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. Elsevier; 2006:2053-2095.
2. Ross DS, et al. Thyroid. 2016;26(10):1343-1421.
3. Vasileiou M, et al; Guideline Committee. BMJ. 2020;368:m41.