Enhanced sialylation of thyrotropin (TSH) prolongs its metabolic clearance rate and thus increases the hormone's in vivo bioactivity. This has been shown for hypothyroid rats and for recombinant human TSH, but there are few data on the sialylation of human serum TSH. The aim of this work was to further study sialylated human serum TSH, its precursors bearing terminal galactose residues, and the role of pharmacological doses of thyrotropin-releasing hormone (TRH) on their secretion under different degrees of primary hypothyroidism. We analyzed serum TSH in patients with subclinical (n = 9) and overt primary hypothyroidism (n = 13) compared with euthyroid individuals (n = 12) and human standard pituitary TSH (IRP 80/558). Blood was drawn before and 30 min after intravenous administration of 200 micrograms TRH, and TSH was purified by immunoaffinity concentration. The content of sialylated (sialo-) TSH and isoforms bearing terminal galactose (Gal-TSH, asialo-Gal-TSH) was measured by Ricinus communis (RCA 120) affinity chromatography in combination with enzymatic cleavage of sialic acid residues. TSH immunoreactivity was measured by an automated second generation TSH immunoassay. Pituitary TSH contained 16.5 +/- 0.8% Gal-TSH. In euthyroid individuals the proportion of Gal-TSH was 14.6 +/- 1.9%, whereas TSH in patients with subclinical and overt primary hypothyroidism contained 23.9 +/- 3.5% (P < 0.05 vs euthyroid individuals) and 21.1 +/- 1.7% Gal-TSH respectively. The mean ratio of asialo-Gal TSH was 23.8 +/- 0.6% for pituitary TSH, 35.7 +/- 4.2% in euthyroid individuals, 48.0 +/- 3.3% in patients with subclinical, and 61.5 +/- 3.8% (P < 0.001 vs euthyroid individuals) in patients with overt primary hypothyroidism. For pituitary TSH the calculated proportion of sialo-TSH was 6.5 +/- 0.2%, for euthyroid individuals 20.3 +/- 2.8%, for patients with subclinical hypothyroidism 24.1 +/- 3.0%, and for patients with overt primary hypothyroidism 40.7 +/- 3.0% (P < 0.001 vs euthyroid individuals). The proportions of Gal-TSH, asialo-Gal-TSH, and sialo-TSH did not differ significantly before and after TRH administration in the individuals studied. Our data show that patients with subclinical and overt primary hypothyroidism have a markedly increased proportion of serum TSH isoforms bearing terminal galactose and sialic acid residues, which may represent a mechanism for the further stimulation of thyroid function. Pharmacological doses of TRH cause an increased quantity of TSH to be released, but do not significantly alter the proportion of sialylated or terminally galactosylated TSH isoforms.