Clinical Significance
TSH with Reflex to Free T4 - For differential diagnosis of primary, secondary, and tertiary hypothyroidism. Also useful in screening for hyperthyroidism. This assay allows adjustment of exogenous thyroxine dosage in hypothyroid patients and in patients on suppressive thyroxine therapy for thyroid neoplasia.

Preferred Specimen(s)
1 mL serum

Minimum Volume
0.7 mL

Collection Instructions
This assay should only be ordered on patients 1 year of age or older. Orders on patients younger than 1 year will have a TSH only performed.

Transport Container
Serum Separator Tube (SST®)

Transport Temperature
Room temperature

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

Includes
For patients 1 year of age or older, Free T4 will be performed at an additional charge (CPT code(s): 84439) when TSH result exceeds age/gender specific reference range.

Patient Preparation
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."

Methodology
Immunoassay (IA)

Reference Range(s)
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/LTSH 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.

Alternative Name(s)
Thyroid Stimulating Hormone with Reflex