Improved Radioimmunoassay for Measurement of Mouse Thyrotropin in Serum: Strain Differences in Thyrotropin Concentration and Thyrotroph Sensitivity to Thyroid Hormone

We report an improved heterologous radioimmunoassay (RIA) for the measurement of thyrotropin (TSH) in mouse serum. The assay components are: antirat thyrotropin (rTSH) serum from the National Hormone and Pituitary Program, a commercial [¹²⁵SI]-labeled rTSH and mouse thyrotropin (mTSH) serum standards produced by dilution of a serum pool from hypothyroid mice with high TSH with a serum pool from mice treated with excess levothyroxine (LT₄) (mTSH-0). Sensitivity was increased by reducing the amount of antibody and tracer and by taking advantage of the disequilibrium technique. Accuracy was greatly improved by the preparation of mouse serum TSH standards. TSH in serial dilutions of individual mice with high TSH of different etiologies paralleled the mTSH standard curve but not that of rTSH or a crude mouse TSH/luteinizing hormone (LH) reference preparation. The high-mTSH-serum standard contained 20 mU TSH per milliliter, measured in a bioassay utilizing a cell line stably transfected with human TSH receptor cDNA, and a relative TSH concentration of 40 ng/mL. The sensitivity of the RIA is 0.01 to 0.02 ng/mL, depending on the quality of the tracer and the preparation of mTSH-0 serum. The intra-assay and interassay coefficients of variations were, respectively: 16% and 27% at 0.04 ng/mL; 6.3% and 8.2% at 0.4 ng/mL; 5.4% and 9.8% at 1.7 ng/mL; 10% and 24% at 4.0 ng/mL. The mean TSH concentration in serum of 60-80-day-old male mice was four-fold higher than that in females of the same age. The assay was able to distinguish differences in serum TSH concentrations in five different strains of mice. Baseline serum TSH concentrations (mean ± SD) of 70-day-old male mice were: 0.143 ± 0.065 ng/mL in the CD-1 strain; 0.229 ± 0.042 ng/mL in C57BL/6 mice; 0.084 ± 0.017 ng/mL in SWR/J mice; 0.133 ± 0.057 ng/mL in NOD SCID mice, and 0.266 ± 0.122 ng/mL in FVB mice. Mean serum thyroxine (T₄) concentrations were also significantly different among the mouse strains but did not correlate with the serum TSH level. Administration of levotriiodothyronine (LT₃) suppressed the serum TSH to a greater degree in mice with higher baseline TSH values. Suppression of the thyroidal radioiodide uptake with LT₃ correlated with that of serum TSH.