How Does Thyroid Hormone Profile Differ on and Off Replacement Treatment?

Untreated patients had median TSH of 1.8 mU/L and median FT4 of 15.5 pmol/L, with 24% of patient results falling outside the 5%/95% percentile boundaries.

• Data derived from 43,394 untreated patients (F 24,386/M 19,008; age <60 32,537/age ≥60 10,857)
• Only single test results were used for untreated patients to minimize comorbidity effects
• Only samples taken in General Practices were used to minimize comorbidity effects
• Cluster analysis used an ellipse with centre on median values for log(TSH) and FT4 with vertex based on 5% and 95% percentile values

Treated patients on levothyroxine had significantly higher median TSH (+30%) and median FT4 (+22%) compared to untreated patients.

• Treated patients had median TSH of 2.3 mU/L compared to 1.8 mU/L in untreated patients (+30%)
• Treated patients had median FT4 of 18.9 pmol/L compared to 15.5 pmol/L in untreated patients (+22%)
• Data derived from 12,006 treated patients (F 9231/M 2775; age <60 5850/age ≥60 6567) with 43,846 test results
• For treated patients, the median value across all their results was used

When assessed against untreated reference limits, 68% of treated patient results fell outside the expected range.

• 68% of treated results fell outside the untreated 5%/95% percentile boundaries
• 22% of treated patients fell outside the treated population's own 5%/95% percentile boundaries
• The displacement outside untreated limits was present for both TSH and FT4 values simultaneously

The proportion of treated patients falling outside untreated reference limits was higher in women than in men.

• 70% of female treated patients fell outside the untreated reference limits
• 63% of male treated patients fell outside the untreated reference limits
• The treated patient cohort was predominantly female (9231 females vs. 2775 males)

The proportion of treated patients falling outside untreated reference limits was slightly higher in younger patients than in older patients.

• 67% of treated patients aged <60 fell outside the untreated reference limits
• 64% of treated patients aged ≥60 fell outside the untreated reference limits
• Age groups were split at 60 years for both treated and untreated populations

The total dataset comprised approximately 290,000 tests on 130,000 individuals drawn from a single laboratory site during 2009-2012.

• Data were extracted from the Salford Royal Hospital Laboratory Information Management System during 2009-2012
• Test requests included a tick box for 'on levothyroxine' (yes or no) to distinguish treated from untreated patients
• After filtering, 12,006 treated patients with 43,846 test results and 43,394 untreated patients with single results were analyzed
• This was a single site study limited to General Practice samples