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With an estimated 120 million prescriptions filled each year, the thyroid medicine levothyroxine (marketed as Synthroid ®) is one of the most popular prescription medicines in the United States. Most patients who suffer from hypothyroidism — a shortage of thyroid hormone, usually caused by a damaged or missing thyroid gland — respond favorably to treatment with this drug.
Nearly 15 percent of patients, however, get only limited benefit from levothyroxine. Their symptoms, such as fatigue, weakness, weight gain, cramps, irritability and often memory loss, persist, even among patients who take this affordable medicine consistently.
On Oct. 23, 2018 the Journal of Clinical Investigation posted an 'in-press preview" of this multi-institutional study, describing how one dysfunctional protein can disrupt the efficacy of this otherwise highly effective treatment. The damage is caused by an inherited mutation in a critical enzyme that puts nearly one out of five patients at risk for not being able to experience the established benefits of levothyroxine.
"Even though they take their medications, many hypothyroid patients continue to have problems," said thyroid specialist Antonio Bianco, MD, PhD, professor of medicine at the University of Chicago and senior author of the study. "They lack energy, they feel unfocused and they have trouble losing weight. They are properly taking thyroid hormone but their problems aren't going away. They get frustrated when they see little change, only limited improvement. Many change physicians multiple times, sometimes more than 10 times."
"In this study, using mice," he added, "we found compelling evidence that the explanation for this problem is a genetic polymorphism that significantly alters the crucial enzyme that metabolizes thyroid hormone. We are seeking ways to fix this."
The primary hormone secreted by the thyroid gland is thyroxine, also known as T4; levothyroxine is the pharmaceutical version of T4. Soon after a patient takes the levothyroxine tablet, T4 is absorbed and enters the circulation, but to gain full biological activity, T4 must be converted to T3 (triiodothyronine). This task is carried out by many cells, including the glial cells in the brain. The conversion relies on an enzyme known as type-2 deiodinase (D2).
Inside these cells, small membrane-wrapped vesicles shuttle D2 back and forth between two intracellular organelles, the endoplasmic reticulum (ER) and the Golgi apparatus. In as many as 20 percent of people who rely on levothyroxine, however, the tiny genetic flaw in D2 causes the shuttling process to go astray. Those patients have a single-nucleotide substitution in the DNA that encodes D2. As a result, one amino acid, threonine, is replaced by a different amino acid, alanine.
This amino-acid switch, known as the Thr92Ala-DIO2 polymorphism, results in a misfolded D2 protein. Because cells recognize it as an abnormal protein, it gets pushed out of the ER and accumulates in distal portions of the Golgi apparatus. Misfolded D2 is less active. It converts some T4 to T3, but the result is a significant overall decrease in the amount of available T3.
"Indeed, the brains of mice carriers of the polymorphism exhibit signs of hypothyroidism," said Bianco. The buildup of misfolded D2 in the ER and Golgi "disrupts the protein homeostasis of the cells, probably complicating things long term for patients who carry this polymorphism."
Patients with the polymorphism, for example, have been reported to be "at higher risk for problems including hypertension, insulin resistance, type 2 diabetes, and multiple cognitive issues. A previous Bianco-led study, performed at Rush University Medical Center, found that African Americans carriers of this polymorphism have a 30 percent higher risk of developing Alzheimer's disease.
These findings led the team to create mice with the threonine-to-alanine polymorphism so they could probe its effects on the brain. "You can't easily study this in humans," Bianco explained, "so we turned to the brains of gene-altered mice."
They were surprised to see that test mice behave in many ways like humans with hypothyroidism. Their activities seemed to coincide with how patients feel. They sleep four times as much during day and night. They stay quiet, don't move around much. They lack the motivation to jump on the spinning wheel and play. They also exhibit memory problems.
A logical explanation is that these mice have brain hypothyroidism, despite having normal thyroid hormone levels in the blood. The researchers tested this theory by screening different parts of the brain for signs of hypothyroidism. The brains of mice carriers of the D2 polymorphism clearly had areas with hypothyroid-like features, specifically in the striatum, pre-frontal cortex and amygdala, areas involved in motivation and decision-making processes.
To confirm this, they treated these animals with T3, and many of the aspects indicating hypothyroid-like behavior were normalized. Unfortunately, long-term treatment of patients with T3 has its drawbacks. T3 has a short half-life. The tablets are rapidly absorbed, which causes levels to spike in the blood. Even at low doses, this can induce palpitations, anxiety, sweating and tightness of the chest.
"We do not know what damage these spikes can cause long-term," Bianco said. "Because people didn't appreciate that T3 was important for hypothyroid patients, the safety studies were never performed." He expects to open a clinical trial of a new agent in 2019.
"We hope that understanding these mechanisms will accelerate development of new therapeutic approaches for the millions of patients with hypothyroidism," the authors conclude, "and provide justification for clinical studies to assess the utility of customization of thyroid replacement therapy based on their Thr92Ala-DIO2 status."