Serum osteocalcin in patients taking L-thyroxine who have subclinical hyperthyroidism.

Serum osteocalcin, an index of osteoblastic activity, is increased in hyperthyroidism. Serum osteocalcin levels are negatively correlated with bone density in patients with overt hyperthyroidism. Osteocalcin levels are also elevated in patients with multinodular goiter and subclinical hyperthyroidism. We therefore measured serum osteocalcin levels in patients taking T4 to determine if they correlated with the degree of TSH suppression. Despite an upward trend in serum osteocalcin measurements with decreasing TSH concentrations, there was no significant difference in serum osteocalcin among groups of patients with normal (0.5-5.0 mu/L), mildly reduced (0.1-0.5 mU/L), or undetectable serum TSH (less than 0.01 mU/L). However, a weak negative correlation was seen between serum TSH and osteocalcin concentrations (r = 0.29, slope = -0.28, P less than 0.05). Osteocalcin did not correlate with either serum free T4 or free T3 concentrations. Serum PTH concentrations were not different among the three patient groups. Our data suggest that osteocalcin is not a useful clinical marker for increased bone turnover in patients with subclinical hyperthyroidism due to T4 therapy. However, the trend towards higher osteocalcin levels in patients with suppressed serum TSH values, and the weak negative correlation between serum TSH and osteocalcin are consistent with findings of reduced bone density in these patients.

Measurement of thyrotropin in clinical and subclinical hyperthyroidism using a new chemiluminescent assay.

A new commercially available chemiluminescent TSH assay demonstrates an 8- to 10-fold increase in sensitivity over a sensitive immunoradiometric assay, allowing increased ability to distinguish partial from more complete thyrotroph suppression. Sera were analyzed from 145 patients who had TSH concentrations below 0.08 mU/L in the immunoradiometric assay. Most patients with overt hyperthyroidism had undetectable TSH concentrations in the chemiluminescent assay. Three groups of patients were identified, in which a large subgroup had undetectable values in the immunoradiometric assay and detectable values in the chemiluminescent assay (12 of 17 patients under treatment for hyperthyroidism who had recently normalized their serum thyroid hormone levels, 33 of 68 patients taking L-T4, and 4 of 8 patients with endogenous subclinical hyperthyroidism). In addition, several patients with undetectable basal and detectable TRH-stimulated TSH values in the immunoradiometric assay had detectable basal TSH values in the chemiluminescent assay. The utility of first generation TSH immunometric assays was their ability to distinguish hyperthyroidism from euthyroidism. The clinical utility of increasingly sensitive TSH assays will be to distinguish degrees of thyrotroph suppression in subclinical hyperthyroidism.

Triiodothyronine (T3) regulation of thyrotropin subunit gene transcription is proportional to T3 nuclear receptor occupancy.

We have investigated the relationship between T3 nuclear receptor occupancy and the T3-mediated responses of TSH subunit gene expression. Hypothyroid mice bearing TtT 97 thyrotropic tumors were injected daily for 12 days with 0-10 micrograms T3/100 g BW, ip. T3 levels were measured in plasma and in tumor nuclei, and the maximal T3-binding capacity of tumor nuclei and the fractional occupancy of T3 nuclear receptors at each dose were calculated. T3-mediated decreases in TSH secretion were half-maximal at a dose of 0.2-0.3 micrograms/100 g BW, which resulted in plasma T3 levels of 0.98-1.2 ng/ml. Responses at the TSH subunit gene levels followed a similar pattern. Transcription of TSH beta and alpha-subunit genes was decreased maximally from 384 to 26 ppm for TSH beta and from 424 to 112 ppm for alpha-subunit. Inhibition of transcription was half-maximal at plasma T3 concentrations of 0.8 and 1.0 ng/ml for TSH beta and alpha-subunit, respectively. The half-maximal effective doses of T3 for decreases in TSH gene transcription were in good agreement with the amount of T3 necessary to saturate 50% of nuclear T3 receptors in the tumor, calculated at 1.07 ng/ml T3. A plot of fractional decrease in TSH subunit gene transcription vs. fractional T3 nuclear receptor occupancy demonstrated a straight line relationship for both TSH beta and alpha-subunit. Thus, the response of both TSH subunit genes to T3, a decrease in TSH beta and alpha-subunit gene transcription, is directly proportional to nuclear T3 receptor occupancy.

Monoclonal antibody to human thyrotropin.

Using the technique of somatic cell fusion, a monoclonal antibody to human TSH (hTSH) has been produced. The monoclonal antibody (anti-hTSH 1/1) has an affinity (Kd = 4.04 x 10(-9) M/liter) for hTSH which is slightly less than that of the polyclonal antisera (Kd = 9.8 x 10(-10) M/liter) derived from the same mouse used for the fusion experiment. The beta-subunit of hTSH displays nearly complete cross-reaction, but has a lower affinity (Kd = 2.6 x 10(-8) M/liter) for the monoclonal antibody. hCG and its alpha- and beta-subunits as well as human LH do not cross-react with this antibody. The high specificity of this antibody will make it useful as an immunofluorescent probe of hTSH-secreting cells as well as in the immunoabsorption and purification of hTSH.