Laboratory Workup for TSH 5.99
For a TSH of 5.99 mIU/L, order a repeat TSH with free T4 in 3-6 weeks before making any treatment decisions, as 30-60% of mildly elevated TSH values normalize spontaneously. 1, 2
Initial Confirmation Testing
Repeat TSH and measure free T4 to distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4). 1 This distinction is critical because:
- Subclinical hypothyroidism = elevated TSH with normal free T4 1, 2
- Overt hypothyroidism = elevated TSH with low free T4 2
The 3-6 week interval is essential because TSH secretion is highly variable and sensitive to acute illness, medications, and physiological factors. 1 A single borderline TSH value should never trigger treatment decisions. 1
Additional Diagnostic Testing to Consider
Anti-TPO antibodies should be measured to identify autoimmune etiology (Hashimoto's thyroiditis), which predicts higher progression risk to overt hypothyroidism (4.3% per year vs 2.6% in antibody-negative individuals). 1 This information influences treatment decisions, particularly for TSH values in the 4.5-10 mIU/L range. 1
Lipid profile may be useful as subclinical hypothyroidism can affect cholesterol levels. 1
Clinical Context That Modifies Testing Urgency
For patients with cardiac disease, atrial fibrillation, or serious medical conditions, consider repeating testing within 2 weeks rather than waiting the full 3-6 weeks. 1
For asymptomatic patients without cardiac risk factors, the standard 3-6 week interval is appropriate. 1
Common Pitfalls to Avoid
Do not order a TRH stimulation test. Modern sensitive TSH assays have made TRH testing obsolete for routine hypothyroidism diagnosis. 1, 2 While older literature suggested TRH testing could identify "sub-biochemical hypothyroidism," 3 current guidelines do not support this approach.
Do not treat based on a single elevated TSH value without confirmation, as transient elevations are common and frequently represent recovery phase thyroiditis. 1, 4
Review recent iodine exposure (such as CT contrast), as this can transiently affect thyroid function tests. 1
Exclude non-thyroidal illness as a cause of TSH elevation, particularly in hospitalized or acutely ill patients, as TSH is frequently suppressed during acute illness and may rebound during recovery. 2
What NOT to Order
T3 levels are not indicated for initial evaluation of hypothyroidism, as T3 remains normal until very late in the disease process. 2 TSH and free T4 are sufficient for diagnosis and classification. 1, 2
Thyroid ultrasound is not routinely indicated unless there is palpable thyroid abnormality, goiter, or concern for nodular disease. 1
Treatment Decision Framework Based on Confirmation Results
If repeat testing confirms TSH >10 mIU/L, levothyroxine therapy is recommended regardless of symptoms, as this carries approximately 5% annual risk of progression to overt hypothyroidism. 1
If repeat testing shows TSH 4.5-10 mIU/L with normal free T4, treatment decisions should be individualized based on symptoms, positive anti-TPO antibodies, pregnancy status, or infertility. 1 Routine treatment is not recommended for asymptomatic patients in this range. 1, 4
If TSH normalizes on repeat testing (which occurs in 62% of cases with mild elevation), 4 no treatment is needed, but consider rechecking in 6-12 months if symptoms develop or risk factors emerge. 1