Thyroid problems linked to increased risk of dementia, study finds Brown University

After a 12-month trial on an older population, Stott et al. observed no effect on executive function as measured by Letter Digit Coding Test. They concluded that levothyroxine did not show any impact on older people with subclinical hypothyroidism. However, the author also mentioned that a higher thyrotropin target could be why there is no beneficial effect 21.

Institutional Review Board Statement

• Neuronal synaptic plasticity, also called long-term potentiation, is a crucial mechanism involved in learning and memory 25,26.
• A vicious loop is created when abnormal βA or phosphorylated tau protein builds up, further promoting redox imbalance (47).
• Thyroid hormones play a significant role in the production and survival of microglia (Lima et al., 2001; Mallet et al., 2002).
• The T4 assay had an analytical sensitivity of 0.30 µg/dL and an intra-assay precision of 4.6% at 8.23 µg/dL.

You might start to experience the side effects of low blood pressure such as dizziness, and lightheadedness, or you might pass out. Lisinopril is used to help lower blood pressure in people who have high blood pressure. If you understand the importance of T4 to T3 conversion, then you can start to see how the measurement of high free T4 in patients with Alzheimer’s may not point to a problem of hyperthyroidism, but a problem of proper T4 to T3 conversion.

The analytical sample for the autopsy study is 143, only five of whom had high and four of whom had low thyrotropin levels. Analysis of covariance adjusted for age was used to compare characteristics of the autopsy cases with participants who dropped out after exam 4 within the thyroid sample. Baseline characteristics of the included autopsy cases did not differ from those in the thyroid sample who dropped out after exam 4 (data not shown). Dementia and its subtypes were identified in a multi-step case-finding procedure, described in detail elsewhere propecia synthroid 14, 33. In brief, all participants underwent neuropsychological screening with the 100-point Cognitive Abilities Screening Instrument (CASI), a measure of global function that has been validated in English and Japanese 31. Diagnosis was based on neuropsychologic testing using the Consortium to Establish a Registry for Alzheimer Disease (CERAD) battery, a neurologic exam and an informant interview.

An experimental study found a 5% increase in GABA levels after a 6-month treatment with LT-4 and a positive correlation of GABA in the medial prefrontal cortex with memory function 17. Deiodination is the initial stage in the activation or inactivation of thyroid hormone, and deiodinases are important in this process (36). The exocyclic deiodination of T4 and its conversion to the active T3 are the primary reactions catalyzed by iodothyronine deiodinase 1 (DIO1) and iodothyronine deiodinase 2 (DIO2), which promote the activation pathway. In the inactivation pathway, iodothyronine deiodinase 3 (DIO3) acts as an essential endocyclic deiodinase by catalyzing the conversion of active T4 and T3 to inactive reverse-T3 and T2 (36).

Does Synthroid Cause Alzheimer’s Disease?

One limitation of the study was that researchers did not factor in the severity of individual cases of hypothyroidism. “One explanation for this could be that these people are more likely to experience greater symptoms from hypothyroidism where treatment was needed,” Weng said. The commentary also noted that levothyroxine tends to be prescribed more frequently to older patients and that several studies have suggested the medication is sometimes prescribed without medical evidence that it’s needed.

• Since previous research had shown that people whose bodies make too much thyroid hormone have a higher risk for dementia, the researchers wanted to find out if the risk was also higher in people taking too much thyroid medication.
• To date, no study has addressed the link between all-cause thyroiditis and dementia and AD.
• Other inclusion criteria were a diagnosis of no dementia and age of 50 years or greater at the baseline assessment, and participation in at least one subsequent clinical assessment.
• A standardized difference of ≤0.1 was deemed an ideal balance, and a standardized difference of ≤0.2 was treated as acceptable balance.

of researchers rate our articles as excellent or good

Each subgroup was stratified into four groups, and the estimates were conducted using the unadjusted model, model 1, and model 2. BACKGROUNDDecreased brain function and dementia are important causes of disability among older people worldwide. While age is still the most critical risk factor, younger people can also have dementia (called early-onset dementia). It is estimated that 2-8% of all dementia cases are early-onset, and therefore early identification of potentially reversible risk factors responsible for decreased brain function and dementia is essential. For the study, researchers analyzed the health records of 7,843 people in Taiwan newly diagnosed with dementia and compared them to the same number of people who did not have dementia. Researchers looked to see who had a history of either hypothyroidism or hyperthyroidism.

1. Evidence Supporting the Link Between Thyroid Disease and Cognitive Impairment

Past research has focused on relationships between dementias and a broad spectrum of drug classes, including estrogens (Slooter et al., 1999), antidepressants (Mayeux and Sano, 1999), antihypertensives (Yasar et al., 2006), and statins (Hayden et al., 2005). Our study attempts to help fill this research void by examining possible associations between use of thyroid medications and AD onset. The search strategy identified 2957 potentially relevant records, of which 1016 were excluded as duplicates. Further, 1765 publications were excluded, as they were reviews, letters or conference abstracts, and independent studies. Finally, 159 articles were excluded owing to inadequate information or lack of a complete publication. A total of 17 cohort studies (6, 7, 8, 9, 10, 11, 12, 13, 15, 21, 22, 23, 24, 25, 26, 27, 28) published from 2000 to 2020 met the inclusion criteria (Fig. 1).

First, due to the cross-sectional character of the study, our analysis could not conclude whether thyroid diseases influenced and contributed to the development or progression of AD or whether they were a result of AD. In fact, several studies have proposed that altered thyroid hormone levels can be a consequence of AD. One study explained that adenohypophysis deterioration due to degenerative changes in the AD brain could lead to a decrease in TRH and TSH production, resulting in low thyroid hormone levels (13). Moreover, another study reported that hypofunction of the adenohypophysis due to circadian rhythm disturbances in AD decreased TSH levels (34). Although it is not elusive whether thyroid dysregulation occurs before or after the onset of AD, treatment with thyroid hormone restoration has been reported to potentially improve cognitive deficits in animal studies (35–37).

(5. Adjusted analyses

At time of examination 4, fasting blood samples were collected and put on ice immediately. Thyroid hormones were assessed in a random sub-sample of 1001 men who participated in examination 4. Several biochemical markers of thyroid function were assayed, including thyrotropin, free thyroxine (fT4) and total thyroxine (T4). Thyrotropin, fT4 and T4 were all measured by chemiluminescence assays on a DPC2000 analyzer (Diagnostic Product Co., Los Angeles, CA). The thyrotropin assay had an analytical sensitivity of 0.004 µU/dL and an inter-assay precision of 3.8% at 1.3 µU/dL.

Does Hormone Supplementation With Levothyroxine Improve Hypothyroid Impaired Cognitive Dysfunction?

For the studies that did not present the median or mean doses of TSH, the midpoint of each category was chosen. The value for the open-ended upper interval was arbitrarily assigned as 1.5 times that of the lower end of the interval. The aggregate generalized least squares for trend (glst) method by Greenland and Longnecker was used for assessing the dose–response relationship between TSH and the risk of dementia (19). We used restricted cubic splines with three knots to explore a potential non-linear association between TSH concentrations and the risk of dementia.