TABLE 6. Calculation of the Free Thyroxine Index (FT4 I) in Situations in Which the Serum TBG Concentration is Abnormal


 

 

 

 

 

High TBG Concentration

Low TBG Concentration

 

Normal

Normal TBG concentration

(Pregnancy, BCP, Heredity)

(Androgens, Heredity)

 

Values

Euthyroid

Hyperthyroid

Hyperthyroid

Euthyroid

Hyperthyroid

Hyperthyroid

Euthyroid

Hyperthyroid

Hyperthyroid

TT4(μg/dL)

5–13

RT3U (%)

25–35

10

15

5

15

15

8

4

10

2

FT4l

5–13

32

38

20

20

30

15

40

50

30

Calculation of

TT4× RT3U

10.6

19

3.6

10

15

4

5.3

16.6

1

 FT4I

Mean normal RT3U

10 × 32/30

15 × 36/30

15 × 20/30

15 × 30/30

15 × 30/30

8 15/30

4 40/30

10 50/30

2 30/30


TBG = thyroxine-binding globulin; TT4 = total thyroxine; RT3U = resin T3 uptake; BCP = birth control pills
(Adapted from Mestman JH: Thyroid diseases in pregnancy. Clin perinatol 12:651, 1985)