Hypothyroidism: a comprehensive review

Post-translational modifications of transthyretin affect the triiodonine-binding potential

Transthyretin (TTR) is a visceral protein, which facilitates the transport of thyroid hormones in blood and cerebrospinal fluid. The homotetrameric structure of TTR enables the simultaneous binding of two thyroid hormones per molecule. Each TTR subunit provides a single cysteine residue (Cys₁₀), which is frequently affected by oxidative post-translational modifications. As Cys₁₀ is part of the thyroid hormone-binding channel within the TTR molecule, PTM of Cys₁₀ may influence the binding of thyroid hormones. Therefore, we analysed the effects of Cys₁₀ modification with sulphonic acid, cysteine, cysteinylglycine and glutathione on binding of triiodothyronine (T3) by molecular modelling. Furthermore, we determined the PTM pattern of TTR in serum of patients with thyroid disease by immunoprecipitation and mass spectrometry to evaluate this association in vivo. The in silico assays demonstrated that oxidative PTM of TTR resulted in substantial reorganization of the intramolecular interactions and also affected the binding of T3 in a chemotype- and site-specific manner with S-glutathionylation as the most potent modulator of T3 binding. These findings were supported by the in vivo results, which indicated thyroid function-specific patterns of TTR with a substantial decrease in S-sulphonated, S-cysteinylglycinated and S-glutathionylated TTR in hypothyroid patients. In conclusion, this study provides evidence that oxidative modifications of Cys₁₀ seem to affect binding of T3 to TTR probably because of the introduction of a sterical hindrance and induction of conformational changes. As oxidative modifications can be dynamically regulated, this may represent a sensitive mechanism to adjust thyroid hormone availability.

Thyroid Disorders in Elderly Patients

Thyroid disorders are common in the elderly and are associated with significant morbidity if left untreated. Typical symptoms may be absent and may be erroneously attributed to normal aging or coexisting disease. Physical examination of the thyroid gland may not be helpful, as the gland is often shrunken and difficult to palpate. Usually only myxedema coma requires levothyroxine parenterally; all other forms of hypothyroidism can be treated with oral levothyroxine. Low-dose levothyroxine should be initiated and increased gradually over several months. In unstable elderly patients with hyperthyroidism, antithyroid medication can quickly produce a euthyroid state. Radioactive iodine therapy is more definitive and is well tolerated, effective, and preferred. Surgical thyroid ablation may be necessary in patients who fail to respond to radioactive iodine therapy and in patients with multinodular goiter. If there is a suspicion of malignant disease, early biopsy or fine needle aspiration for cytology should be considered.