• Low Thyroid: Treatment with Only Synthroid Enough?

    on Jul 5th, 2017

As a bio-identical hormone doctor, I frequently answer questions about low thyroid treatment (hypothyroidism).  New research suggests that certain patients with low thyroid may benefit from both T4 and T3 treatments. In addition, several everyday environmental toxins may trigger or worsen hypothyroidism.

What is Hypothyroidism?

Hypothyroidism is a condition in which the thyroid gland, which regulates the metabolic rate, is slow in making and secreting thyroid hormones. About 8-10 million Americans have hypothyroidism, also called having a low thyroid or an underactive thyroid.

There are two kinds of thyroid hormones: T4, which is the storage hormone, and T3, which is the active hormone. The thyroid gland produces all of the T4 and some T3. Most hormones released by the thyroid gland is T4, which is converted to T3 at the tissue level depending on the need of each tissue.  Therefore, most T3 throughout the body is not necessarily produced by the thyroid gland.Thyroid Treatment Synthroid

Treating Hypothyroidism

Until recently, the thinking behind the treatment of low thyroid has been this: If the body can convert T4 to T3, then giving T4 should suffice. Resulting in patients with hypothyroidism have been treated only with pharmaceutical-grade T4 (levothyroxine).

This treatment has not included T3. The rationale has been that because the body can convert T4 into T3, it is not necessary to treat hypothyroidism using T3. Thyroid researchers have studied mice with low levels of T3.  They challenged the thyroid system of the mice by giving T4 and found that even though T4 levels increased, T3 levels stayed the same. This posed two new questions: 1) Do hypothyroid patients, given only T4, have a normal T3? 2) Can T3 be balanced by giving T4 to a patient who does not have normal thyroid function?

The short answer: It depends on the genetic expression of a specific enzyme, DIO2.

Genetic Testing to Help Determine Treatment

DIO2 is the enzyme that transforms T4 to T3, and about 15-20% of the population has a genetic mutation (called a polymorphism) in this enzyme that changes one amino acid in the DOI2 protein. Why is this important? A study in the UK showed that those with the DIO2 polymorphism benefitted from treatment that included both T3 and T4.  The most common prescription for hypothyroidism is Synthroid, which is T4 only.  Patients with the DIO2 mutation will not do well on treatment with Synthroid only because T4 is not properly converted to T3 by the DIO2 enzyme.

This gene test can be ordered through 23andMe, a popular direct-to-consumer DNA genetic testing, and analysis laboratory. If the 23andMe test shows that you have a CC genotype, then you will have decreased conversion of T4 to T3 outside of the thyroid gland. In this case, the best treatment for low thyroid function is supplementation with both T4 and T3.

Different Tissues in the Body have Different Concentrations of DIO2

Thyroid hormone needs differ depending on the organs (e.g., muscle, heart, liver, kidney, and the brain).  The brain has high concentrations of DIO2, which is the only activating enzyme that converts T4 to T3, whereas other tissues have other converting enzymes such as DIO1 and DIO3.  Given that the brain has high concentrations of DIO2, it tells us that T3 is very important to the brain. Research has shown that people with DIO2 mutations with hypothyroidism had impaired psychological well-being if treated only with T4; reversed when patients received treatment with both T3 and T4.

Preventing Hypothyroidism: What You Can Do

A number of environmental toxins can contribute to a low thyroid condition. Not every person’s body will respond to these toxins the same way genetically, being mindful of the following—and avoiding these toxins whenever possible—might help prevent hypothyroidism:

Wondering if you have low thyroid function—or looking for holistic ways to approach it?  Book an appointment with me for a bio-identical hormone health assessment and personalized treatment plan.


Panicker, V. Genetics of thyroid function and disease. Clin Biochem Rev. 2011 Nov; 32(4): 165-175.

Gustafson, C, Antonio C. Bianco: Is T4 enough for patients with hypothyroidism dysfunction? Integrative Medicine. 2014; 13(3): 20-22.

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