Regardless of whether there are T4-specific receptors, or receptors that can be triggered by T4 or T3 (or something else!), there is an issue which is important and documented quite widely.
Our blood stream does not circulate throughout our bodies evenly. Some parts are relatively isolated. The most talked-about is probably the blood-brain barrier (BBB).
For thyroid hormone, any thyroid hormone, to get from the bloodstream into the brain it needs to be actively transported across the BBB. Several active transporter mechanisms have been identified but there is still a lack of complete clarity about them. At one time, it was claimed that T3 cannot get into the brain at all (from the bloodstream) – that appears to have been dismissed, if only from the evidence of people with no thyroid activity who survive, often very well, on T3-only!
Could it be that in addition to acting as a storage reservoir, T4 also helps to get sufficient thyroid hormone to parts which are relatively distant, in a blood flow sense? (The BBB makes the brain more “distant” by requiring transporter mechanisms.) For example, by how much is T3 level reduced between the heart and the toes? The implication of this thought is that distant tissue will likely have to do more of its own, local conversion, because the blood arriving is already relatively depleted of T3. Certainly, hair follicles do. The brain does. Do other distant tissues like finger and toenails?
Perhaps there is a germ of an idea as to why people taking a significant amount of T3 often seem to need what appear relatively large doses?
Have to end my meandering thoughts here – and put a few links. The first one is available in full. The other two are, unfortunately, behind paywalls.
Molecular aspects of thyroid hormone transporters, including MCT8, MCT10, and OATPs, and the effects of genetic variation in these transporters
jme.endocrinology-journals....
Mol Cell Endocrinol. 2017 Jan 24. pii: S0303-7207(17)30042-4. doi: 10.1016/j.mce.2017.01.029. [Epub ahead of print]
The ups and downs of the thyroxine pro-hormone hypothesis.
Galton VA1.
Author information
• 1Department of Physiology and Neurobiology, The Geisel School of Medicine at Dartmouth, 1 Medical Center Drive, Lebanon, NH 03756, USA. Electronic address: Val.galton@dartmouth.edu.
Abstract
Thyroxine (T4) is the major thyroid hormone in the thyroid gland and the circulation. However, it is widely accepted on the basis of abundant evidence that 3,5,3'-triiodothyronine (T3) is responsible for most, if not all, of the physiological effects of TH in extrathyroidal tissues, and T4 functions as the pro-hormone. Whether T4 has any intrinsic activity per se or is merely a pro-hormone that must be converted to T3 in order to exert any TH action has yet to be resolved. Although there are some physiological actions of T4 that are mediated by receptors at the cell membrane (non-genomic effects), the vast majority of the physiological effects of the THs identified to date involve the binding of T3 to specific nuclear receptors to regulate gene expression (genomic effects). This review examines how the role of T4 in genomic TH action has been viewed and debated during the hundred years since it was first isolated in 1914.
Copyright © 2017. Published by Elsevier B.V.
KEYWORDS:
3,5,3'-triiodothyronine; Deiodinases; Thyroid hormone action; Thyroid hormone receptors; Thyroxine
PMID: 28130114
DOI: 10.1016/j.mce.2017.01.029
ncbi.nlm.nih.gov/pubmed/281...
Mol Endocrinol. 2014 Apr;28(4):534-45. doi: 10.1210/me.2013-1359. Epub 2014 Feb 19.
Identification of a new hormone-binding site on the surface of thyroid hormone receptor.
Souza PC1, Puhl AC, Martínez L, Aparício R, Nascimento AS, Figueira AC, Nguyen P, Webb P, Skaf MS, Polikarpov I.
Author information
• 1Institute of Chemistry (P.C.T.S., L.M., R.A., M.S.S.), State University of Campinas-UNICAMP, Campinas, Sao Paulo, Brazil; Institute of Physics of São Carlos (A.C.P., A.S.N., P.W., I.P.), University of São Paulo-USP, São Carlos, Sao Paulo, Brazil; National Laboratory of Biosciences (A.C.M.F.), CNPEM, Campinas, Sao Paulo, Brazil; University of California Medical Center (P.N.), Diabetes Center, San Francisco, California; and Genomic Medicine (P.W.), Houston Methodist Research Institute, Houston, Texas.
Abstract
Thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily of ligand-activated transcription factors involved in cell differentiation, growth, and homeostasis. Although X-ray structures of many nuclear receptor ligand-binding domains (LBDs) reveal that the ligand binds within the hydrophobic core of the ligand-binding pocket, a few studies suggest the possibility of ligands binding to other sites. Here, we report a new x-ray crystallographic structure of TR-LBD that shows a second binding site for T3 and T4 located between H9, H10, and H11 of the TRα LBD surface. Statistical multiple sequence analysis, site-directed mutagenesis, and cell transactivation assays indicate that residues of the second binding site could be important for the TR function. We also conducted molecular dynamics simulations to investigate ligand mobility and ligand-protein interaction for T3 and T4 bound to this new TR surface-binding site. Extensive molecular dynamics simulations designed to compute ligand-protein dissociation constant indicate that the binding affinities to this surface site are of the order of the plasma and intracellular concentrations of the thyroid hormones, suggesting that ligands may bind to this new binding site under physiological conditions. Therefore, the second binding site could be useful as a new target site for drug design and could modulate selectively TR functions.
PMID: 24552590
DOI: 10.1210/me.2013-1359
How do we get enough T3 into thyroid hormone receptors? 
Feedback loop TSH/T4/T3
