What is the appropriate management for a patient with hypothyroidism indicated by low Total T4 (thyroxine) and Free T4, and low Total T3 (triiodothyronine) and elevated Free T3 levels?

Management of Hypothyroidism with Discordant Thyroid Function Tests

The patient's laboratory values indicate hypothyroidism with low total T4, low free T4, low total T3, but elevated free T3, requiring levothyroxine replacement therapy with careful monitoring of TSH and free T4 levels to guide dosing adjustments.

Understanding the Patient's Thyroid Status

• The patient's laboratory values show low total T4 (6.19), low free T4 (0.9), low total T3 (0.64), and paradoxically elevated free T3 (2.41), which is consistent with primary hypothyroidism with possible assay interference affecting the free T3 measurement 1, 2
• Discordant thyroid function tests (particularly the elevated free T3 with low total T3) suggest possible laboratory interference, which can occur due to heterophile antibodies or other factors affecting the assay 2, 3
• When there is discrepancy between thyroid function test results and clinical presentation, interference should be considered to avoid erroneous diagnosis and potentially harmful treatment 2

Diagnostic Approach

• TSH is the most sensitive indicator of primary hypothyroidism and should be measured to confirm the diagnosis and monitor treatment 4
• Free T4 measurement is essential to further narrow the diagnosis when TSH is abnormal 4
• The discordant free T3 result should not guide clinical decision-making as T3 levels bear little relation to thyroid status in patients with hypothyroidism 1
• Consider repeating thyroid function tests using a different laboratory platform to confirm results, as different assay methods may be affected differently by interfering factors 3

Treatment Recommendations

• Initiate levothyroxine replacement therapy based on the low T4 values, which clearly indicate hypothyroidism despite the discordant free T3 level 5
• For adult patients with primary hypothyroidism, the typical starting dose is 1.6 mcg/kg/day of levothyroxine 5
• For this 45-year-old female patient, start at a lower dose (e.g., 50-75 mcg daily) and titrate upward based on TSH and clinical response 5
• Levothyroxine is the preferred replacement therapy as it produces more consistently physiological concentrations of T3 6

Monitoring Recommendations

• After initiating therapy, monitor TSH and free T4 levels after 6-8 weeks and adjust dosage as needed 5
• Do not use T3 levels to guide therapy as they do not reliably reflect thyroid status in patients on levothyroxine replacement 1
• The goal of therapy is to normalize TSH levels and improve clinical symptoms 5
• Once stable, evaluate clinical and biochemical response every 6-12 months and whenever there is a change in clinical status 5

Special Considerations

• Investigate for potential causes of primary hypothyroidism, including Hashimoto's thyroiditis, which is common in women of this age group 7
• Be aware that laboratory interference can lead to misleading free T3 and free T4 results, as seen in this case with the discordant free T3 level 3
• If clinical presentation does not match laboratory findings after treatment initiation, consider repeating tests using a different assay platform 3
• Persistent clinical evidence of hypothyroidism despite adequate replacement dose may indicate inadequate absorption, poor compliance, or drug interactions 5

Potential Pitfalls

• Do not be misled by the elevated free T3 level, as this is likely due to assay interference rather than true hyperthyroidism 1, 3
• Avoid overtreatment based on T3 levels, as normal T3 levels can be seen in over-replaced patients and do not reliably reflect thyroid status 1
• Be cautious about interpreting thyroid function tests in isolation; always correlate with clinical presentation 2
• Remember that over-replacement with levothyroxine occurs in a substantial proportion of patients and can have harmful effects 7