Volume 4 Issue 1                                              February/March 1999

New Thyroid Function Test Cascade

The Regional Laboratory Alliance is introducing a redesigned test requisition that includes a new thyroid test cascade.  This cascade is recommended for thyroid screening, but not for management of patients being treated for thyroid disease. Testing will begin with a sensitive TSH.  If TSH is normal, no further testing will be done. Free T₄ (fT₄) will automatically be performed if TSH is either high or low.  If TSH is decreased and fT₄ is normal, total T₃ will be measured to detect T₃ toxicosis. 

The new thyroid cascade follows the recommendations of the American Thyroid Association, the American College of Physicians, and the College of American Pathologists. Testing begins with TSH because it is the most sensitive indicator of thyroid metabolism in patients with normal pituitary gland function. TSH usually shifts out of the normal range sooner than T₄ or fT₄.  Linear changes in fT₄ concentration result in logarithmic changes in TSH secretion.  Approximately 85% of patients have normal TSH values and do not require further testing. 

Decreased TSH values suggest either primary hyperthyroidism, exogenous hyperthyroxinemia, or the effect of glucocorticoids.  Subsequent measurement of fT₄ serves as an index of the severity of hyperthyroidism, because TSH levels can be suppressed even by small increases in fT₄.  If fT₄ is normal, total T₃ is measured to detect T₃ toxicosis. 

Increased TSH levels suggest primary hypothyroidism.  However, the rare conditions of a TSH secreting pituitary adenoma or thyroid resistance may be present.  If fT₄ levels are low, the patient is hypothyroid. Patients with TSH levels greater than 10 uU/mL, but normal fT₄ levels, are classified as primary hypothyroidism.  Subclinical hypothyroidism is usually defined as a TSH level between 5 and 10 uU/mL and normal fT₄. Patients with pituitary disease or neuropsychiatric disorders may require more extensive testing. 

This cascade is not recommended for patients being treated for thyroid disease, because it may actually result in excessive testing.  TSH alone is sufficient to monitor patients receiving thyroid replacement therapy for primary hypothyroidism.  Therapy is based on the TSH value and not Free T4.  In patients receiving antithyroid drugs for hyperthyroidism, fT₄ or T₇ can be used to manage therapy while TSH is suppressed.  Later on, TSH is sufficient to manage therapy. 

NEW LEE’S SUMMIT PATIENT SERVICE CENTER

On March 1, the Regional Laboratory Alliance Lee’s Summit Patient Service Center relocated to:

500 NE 291 Highway

Lee’s Summit, MO 64086

Phone (816) 554-1518

Fax (816) 554-8710

Mon - Fri 8:00 - 5:00