Do thyroid receptors matter? There is often very little if any discussion regarding thyroid receptors. Thyroid receptors are what thyroid hormone (primarily T3) binds to inside your cells. There are also receptors for thyroid hormone on the outside of cells, but that discussion is for another day.
The primary thyroid receptors inside cells are classified as thyroid receptor alpha (TRA) and thyroid receptor beta (TRB).
Today I want to discuss thyroid receptor beta (TRB). Thyroid receptor beta has two forms (called isoforms), TRB1 and TRB2.
Who cares right? Well you should if you are struggling with hypothyroidism and chronic hypothyroid symptoms, especially if you are on thyroid medication and still symptomatic.
TRB2 is found primarily in the central nervous system, hypothalamus, and pituitary gland. TRB1 is found primarily in peripheral tissues.
What very few people seem to understand is that all thyroid receptors do NOT have the same sensitivity to thyroid hormone. Even though thyroid receptor Beta 1 and Beta 2 are very similar, their sensitivity to thyroid hormone is significantly different.
This is likely a protective mechanism. When the brain (CNS), hypothalamus, and pituitary register an influx of T3/4, they can shut down further thyroid production before problems occur of excess thyroid hormone production.
Because the CNS, hypothalamus and pituitary TRB2 receptors are 10x more sensitive, they may become satisfied and shut down thyroid hormone production well before peripheral tissues are satisfied, causing peripheral cellular hypothyroidism.
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But what if you are on thyroid hormone replacement therapy?
If thyroid hormone replacement is given and the CNS, Hypothalamus, and pituitary are satisfied with a very small dose of thyroid hormone, TSH levels will drop and not necessarily be reflective of T3 peripheral tissue status.
Just because your TSH levels are normal or low does not mean your peripheral tissue need for T3 has been satisfied. If peripheral tissue T3 levels are insufficient it can lead to peripheral cellular hypothyroidism and chronic hypothyroid symptoms.