[Sex hormone binding globulin (SHBG), a new metabolic in vitro thyroid function test].

SHBG (sex hormone binding globulin) is a carrier protein for the sex hormones testosterone and estradiol with a molecular weight of about 95000 dalton. It can be used as a metabolic test of thyroid function. SHBG was measured by the adsorption method of Mickelson and Petra; the SHBG contained in serum is incubated with 3H-5alpha-dihydrotestosterone and adsorbed to a cellulose filter. Thirty-eight female patients with hyperthyroidism before treatment had markedly elevated levels of SHBG (x +/- SD: 4.85 +/- 2.4 microgram DHT/100 ml) compared with normal controls (1.50 +/- 0.57; p is less than 0.001). A good correlation between the thyroid hormones and SHBG could be domonstrated which was better for T3 than for T4:r =0.76 (p is less than 0.001) for T3 and r= 0.65 (p is less than 0.001) for T4. This agrees with the clinical finding that the circulating T3 level is a better index of the metabolic severity of thyrotoxicosis than T4. After radioiodine treatment SHBG returns to normal values in euthyroid patients (1.38 +/- 0.8; n = 15) and remains elevated in persistent hyperthyroidism (3.99 +/-1,6; n = 67). Even in patients with persistent biochemical hyperthyroidism who are completely euthyroid on clinical examination, SHBG remains high. Despite lack of evidence of clinical hyperthyroidism, this metabolic test demonstrates the biologic significance of merely biochemical hyperthyroidism. Estimation of SHBG as a metabolic thyroid function test in vitro is of special value for the evaluation of patients showing discrepancies between the clinical and biochemical states and for borderline hyperthyroidism.