Thyroid function tests (TFTs) are overused. One potential reason for this is the wide variety of symptoms associated with thyroid disease, leading to the inclusion of thyroid disease on many differentials. But patients without known thyroid disease who complain of only 1 or 2 of these symptoms may be no more likely to have abnormal TFTs than patients who are asymptomatic.
The total number of signs and symptoms (rather than one particular sign or symptom) appears to be a more reliable indicator of the presence of thyroid disease. A systematic review found the following positive likelihood ratios (LR+) based on the number of signs or symptoms present:
- 1 to 2 signs or symptoms: LR+ of 0.11 to 0.2
- 3 to 4: LR+ of 0.74 to 1.14
- ≥5: LR+ of 6.75 to 18.6
According to a Things We Do For No Reason article on the use of thyroid testing in hospitalized patients, if an adult patient (prevalence of undiagnosed hypothyroidism estimated to be 0.6%) has constipation and fatigue (LR+ 0.2) then the pretest probability would be approximately 0.1%. If the TSH level results between 6.7 and 20 mIU/L (LR+ 0.74), the posttest probability of thyroid disease would remain only 0.1%. Alternatively, a patient with five symptoms consistent with hypothyroidism (LR+ 18.6) would have a pretest probability of 10%. If the TSH level results >20 mIU/L (LR+ 11.1), then the posttest probability of hypothyroidism would be 55%.
National guidelines recommend a TSH-centered approach when ordering TFTs for screening purposes (i.e., screen patients only with TSH or TSH-reflex). In other words, T3 (triiodothyronine) and T4 (thyroxine) are often inappropriately ordered when initially evaluating a patient for thyroid disease. T3 can be particularly problematic:
- There are few situations in which T3 testing will change clinical management (e.g., autonomously functioning thyroid nodules, distinguishing between hyperthyroidism and thyroiditis)
- The American Thyroid Association recommends to avoid checking free T3 because of its high rate of variability