What’s with low free T3 thyroid hormone levels?

It’s a source of repeated frustration: failure to address and remedy low free T3 thyroid hormone.

Recall that there are two primary thyroid hormones: T4 and T3, the “4” and “3” referring to the number of iodine atoms per thyroid hormone molecule. The thyroid obviously therefore requires iodine to manufacture thyroid hormones. The majority of thyroid hormone produced by the thyroid is the T4 form, while T4 is converted to the active thyroid hormone, T3, in the periphery, i.e., muscle, heart, and other organs. T4 is long-acting, lasting for days, while T3 lasts a much shorter period of time. T4 therefore serves as a virtual repository for continual sourcing of T3.

It means that any factor that interferes with conversion of T4 to T3, performed by enzymes called “deiodinases” that remove one iodine atom can lead to symptoms of inadequate thyroid hormone levels, i.e., hypothyroidism. It is this phenomenon–impaired conversion of T4-to-T3–that is responsible for the many people who take the T4 thyroid hormone (levothyroxine, Synthroid, others) yet continue to experience hypothyroid symptoms such as persistently cold hands and feet, fatigue, dry skin, need for excessive sleep, and water retention. Less commonly, conversion of T4 to T3 is somehow diverted into producing an ineffective mirror image molecule of the T3 thyroid hormone, so-called “reverse T3.” Higher levels of reverse T3 can also yield persistent hypothyroid symptoms. In both situations, other thyroid measures such as TSH and free T4 can appear to be at optimal ranges, yet hypothyroid symptoms can persist.

Why would there be impaired conversion of T4 to T3? Perhaps not all the reasons have been identified, but we know that there are indeed reasons for this phenomenon that include:

• Exposure to ubiquitous industrial chemicals–There is abundant evidence that industrial chemicals such as bisphenol A, perfluroooctanoic acid, polychlorinated biphenyls and others interfere with thyroid hormone metabolism, including conversion of T4 to T3. Studies such as this and others have shown that the deiodinadse enzyme is inhibited by BPA. Phthalates and parabens, common ingredients in toiletries such as hand creams and hair conditioners, also interfere with thyroid hormone status. Because endocrinologists are largely unaware of the wealth of evidence that is coming from the toxicological world, they continue to deny that such phenomena exist and insist on prescribing T4 only without addressing T3. Obviously, avoidance of such industrial chemicals would be ideal, but they are ubiquitous, found in water, air, soil, plastics, etc. and can persist for years in the body. We therefore resort to a workaround: add T3 to the mix.
• Dysbiotic changes in bowel flora–Dysbiosis is the rule in the modern world, all of us having been exposed to antibiotics, emulsifying agents, synthetic sweeteners like aspartame and sucralose, stomach acid-blocking drugs, and statin drugs. Disrupted composition of bowel flora influences thyroid hormone metabolism.
• Small intestinal bacterial overgrowth, SIBO–This is a more severe degree of dysbiosis in which unhealthy bacterial species such as Klebsiella and Pseudomonas have proliferated, outmuscled desirable species, then ascended up the ileum, jejunum, duodenum, and stomach, yielding trillions of microbes living and dying in a short space of time. Hypothyroidism increases the likelihood of developing SIBO. SIBO can also add to thyroid dysfunction, since SIBO microbes can interfere with thyroid hormone metabolism and the endotoxemia (the flood of bacterial breakdown products into the bloodstream) of SIBO can also cause thyroid gland inflammation.
• Genetic variation in deiodinase genes–Not everyone has equal capability to convert T4 to T3, as genetic variants enter into the equation, also, impairing some people’s ability to have optimal thyroid status with T4 administered alone.

Industrial chemicals, dysbiosis, SIBO–these are not rare, but are ubiquitous, affecting virtually everyone in the modern world. We can, of course, address dysbiosis and SIBO, but complete avoidance or purging your body of industrial chemicals is not presently possible, nor can you alter your genetics. If your doctor continues to insist that your feeling cold, fatigue, and failure to lose weight are meaningless while you only have your T4 thyroid hormone addressed, then it’s time to find a new doctor.