Studies estimate that 5-10% of people still experience symptoms of hypothyroidism (HD) despite being on thyroxine replacement therapy. Unfortunately, in these situations, individuals often feel dismissed or not listened to and may resort to less regulated or potentially dangerous treatments like animal desiccated thyroxine or high-dose iodine.
Why Might Symptoms Persist Despite Thyroxine?
There are two main theories:
1️⃣ Poor Conversion of T4 to T3: Normally, the thyroid produces 80-90% T4 and 10-20% of the more biologically active T3. We typically replace T4 because it converts to T3 in tissues via an enzyme called DIO2. Around 12-36% of the population have minor variations (polymorphisms) in DIO2, which could result in less T3 and persistent symptoms. Studies on replacing T3 have shown mixed results; some people feel better, worse, or no different. Importantly, those who feel better do not correlate with T3 levels on blood tests. The assay used to test T3 is not very accurate, and T3 is converted in tissues, not the blood. T3 also needs to be taken several times per day and can cause heart issues, so it is not routinely recommended.
2️⃣ Underlying Autoimmune Process: Some studies suggest that the underlying autoimmune process causes the symptoms. Comparisons between people who’ve had their thyroid removed for a nodule/cancer (and therefore need thyroxine) and those with Hashimoto’s disease show that symptoms like fatigue and quality of life scores are worse in those with Hashimoto’s. A somewhat controversial Norwegian study of 150 people with persistent symptoms randomized participants to thyroidectomy or not. Several symptoms improved in those who had the thyroidectomy, and antibodies went down. However, as there was no mock surgery for the control group, it’s hard to distinguish the placebo effect.
Acknowledging the Issue
Regardless of the cause, acknowledging this issue is important and working together as a team is crucial, even if medicine doesn’t have all the answers yet. There is also some weak evidence for selenium helping HD symptoms, and it’s important to screen for other common autoimmune issues like celiac disease.