Thyroid-stimulating hormone (TSH): measurement of intracellular, secreted, and circulating hormone in Xenopus laevis and Xenopus tropicalis

Thyroid-stimulating hormone (TSH) is an important regulator of the hypothalamic-pituitary-thyroid (HPT) axis in Xenopus laevis. To evaluate the role of this hormone on developing tadpoles, immunologically-based Western blots and sandwich ELISAs were developed for measuring intracellular (within pituitaries), secreted (ex vivo pituitary culture), and circulating (serum) amounts. Despite the small size of the tadpoles, these methods were able to easily measure intracellular and secreted TSH, and circulating TSH was measurable in situations where high levels were induced. The method was validated after obtaining a highly purified and enriched TSH sample using anti-TSH-β antibodies conjugated to magnetic beads. Subsequent mass-spectrometric analysis of the bands from SDS-PAGE and Western procedures identified the presence of amino acid sequences corresponding to TSH subunits. The purified sample was also used to prepare standard curves for quantitative analysis. The Western and ELISA methods had limits of detection in the low nanogram range. While the majority of the developmental work for these methods was done with X. laevis, the methods also detected TSH in Xenopus tropicalis. To our knowledge this is the first report of a specific detection method for TSH in these species, and the first to measure circulating TSH in amphibians. Examples of the utility of the methods include measuring a gradual increase in pituitary TSH at key stages of development, peaking at stages 58-62; the suppression of TSH secretion from cultured pituitaries in the presence of thyroid hormone (T4); and increases in serum TSH following thyroidectomy.