Identification of thyroid hormone residues on serum thyroglobulin: a clue to the source of circulating thyroglobulin in thyroid diseases

Thyroglobulin (Tg) present in the serum of normal individuals and patients with thyroid disorders could be partly newly synthesized non-iodinated Tg a nd partly Tg containing iodine and hormone residues originating from the lu men of thyroid follicles. With the aim of examining the contribution of the latter source of Tg to the elevation of serum Tg concentration in thyroid pathophysiological situations, we devised a procedure to identify thyroxine (T4) and tri-iodothyronine (T3) residues on Tg from unfractionated serum. A two-step method, based on (i) adsorption of Tg on an immobilized anti-hum an Tg (hTg) monoclonal antibody (mAb) and (ii) recognition of hormone resid ues on adsorbed Tg by binding of radioiodinated anti-T4 mAb and anti-T3 mAb , was used to analyze serum Tg from patients with either Graves' disease (G D), subacute thyroiditis (ST) or metastatic differentiated thyroid cancer ( DTC). Purified hTg preparations with different iodine and hormone contents were used as reference. Adsorption of purified Tg and serum Tg on immobiliz ed anti-hTg mAb ranged between 85 and 90% over a wide concentration range. Labeled anti-T4 and anti-T3 mAbs bound to adsorbed purified Tg in amounts r elated to its iodine content. Tg adsorbed from six out of six sera from ST exhibited anti-T4 and anti-T3 mAb binding activities. In contrast, signific ant mAb binding was only observed in one out of eight sera from untreated G D patients and in 1 out of 13 sera from patients with DTC. The patient with DTC, whose serum Tg contained T4 and T3, represented a case of hyperthyroi dism caused by a metastatic follicular carcinoma. In conclusion, we have id entified, for the first time, T4 and T3 residues on circulating Tg. The pre sence of Tg with hormone residues in serum is occasional in GD and DTC but is a common and probably distinctive feature of ST.