Thyroid-stimulating hormone (TSH), like the other pituitary glycoprotein hormones, is produced and secreted as a mixture of isoforms, the majority of which represent differences in oligosaccharide structure and possess different bioactivity. When samples are quantified simultaneously by immunometric assay and bioassay, the ratio between bioactivity (B) and immunoreactivity (I) may serve as an index of the overall potency of TSH. Variations of the TSH B/I ratio have been documented in both physiological and pathological conditions associated with alteration of the two most important mechanisms controlling TSH synthesis and secretion, i.e. TRH release and the thyroid hormone feedback system. Major examples of this assumption are the low TSH bioactivity found in samples from patients lacking TRH and thus bearing a hypothalamic hypothyroidism, and the enhanced bioactivity that is invariably found in TSH from patients with thyroid hormone resistance. Moreover, variations of TSH bioactivity have been recorded in normal subjects during the nocturnal TSH surge, in normal fetuses during the last trimester of pregnancy, in patients with primary hypothyroidism and in patients with TSH-secreting pituitary adenoma and non-thyroidal illness. In conclusion, the secretion of TSH molecules with altered bioactivity plays an important pathogenetic role in various thyroid disorders, while in some particular physiological conditions the bioactivity of TSH may vary in order to adjust thyroid hormone secretion to temporary needs.