The processes and pathways mediating the intermediary metabolism of carbohydrates, lipids, and proteins are all affected by thyroid hormones (THs) in almost all tissues. Particular attention has been devoted by scientists to the effects of THs on lipid metabolism. Among others, effects related to cholesterol, lipid handling, and cardiac performance have been the subject of study. Many reports are present in the literature concerning the calorigenic effect of THs, with most of them aimed at identifying the molecular basis of this effect. However, at the moment the mechanism(s) underlying the metabolic effects of THs remain to be elucidated. THs exert most of their effects though TH receptors (TRs). However, some effects of THs cannot be explained by a nuclear-mediated pathway, and recently an increasing number of nonnuclear actions have been described, which can provide a regulatory system of which the effects differ from those mediated on the transcriptional level by TRs. Some of the TH derivatives (naturally occurring metabolites and analogs) possess biological activities. TH-related biological effects have been described for physiological products such as tetraiodothyroacetic acid (Tetrac) and triiodothyroacetic acid (Triac) (via oxidative deamination and decarboxylation of thyroxine [T4] and triiodothyronine [T3] alanine chain), 3,3',5'-triiodothyronine (rT3) (via T4 and T3 deiodination), 3,3'-diiodothyronine (3,3'-T2) and 3,5-diiodothyronine (T2) (via T4, T3, and rT3 deiodination), and 3-iodothyronamine (T1AM) and thyronamine (T0AM) (via T4 and T3 deiodination and amino acid decarboxylation), as well as for TH structural analogs, such as 3,5,3'-triiodothyropropionic acid (Triprop), 3,5-dibromo-3-pyridazinone-l-thyronine (L-940901), N-[3,5-dimethyl-4-(4'-hydroxy-3'-isopropylphenoxy)-phenyl]-oxamic acid (CGS 23425), 3,5-dimethyl-4[(4'-hydroxy-3'-isopropylbenzyl)-phenoxy] acetic acid (GC-1), 3,5-dichloro-4[(4-hydroxy-3-isopropylphenoxy)phenyl] acetic acid (KB-141), and 3,5-diiodothyropropionic acid (DITPA). Most of these compounds have interesting properties: counteracting lipid accumulation, reducing cholesterol level, and increasing lipid metabolism without cardiotoxic effects. Hopefully, further studies on basic mechanisms of such compounds will be harbingers of more knowledge on the metabolic effects of TH derivatives and on their possible clinical application.