Table 2.

The ability of thyroid function test to detect deviations from the euthyroid state among euthyroid participants and patients with subclinical (mild) hypothyroidism. Calculations are based on the measurements listed in Table 1

	TSH	TT4	fT4 index	fT4
Laboratory reference range:	0.3–4.5 mIU/L	60–140 nM	70–140 nM	12–22 pM
Samples within the reference range:
Euthyroid, % (n)	100 (180)	99.4 (179)	97.8 (176)
Mildly hypothyroid, % (n)a	12.9 (31)	97.1 (233)		78.3 (188)
Sensitivity, %	87.1	2.9	21.7
Specificity, %	100	99.4	98.9
PPV, positive predictive value, %	100	87.5	96.3
NPV, negative predictive value, %	85.3	43.4	48.8

TSH Thyrotropin, TT4 Total thyroxine, fT4 Free thyroxine

^a One participant had subsequent measures of TSH and T4 within the reference range. Without this participant, 8.3% of TSH and 96.9% of T4 measurements were within the population-based reference range in this group