Thyroid hormones (thyroxine and it’s prohormone, triiodothyronine) are important determinants of the basal metabolic rate of every cell in the body. Structurally, these compounds are iodinated derivates of the amino acid l-tyrosine.
The beneficial effects of thyroid on cognitive function are well-characterized. Major side effects of hypothyroidism (low circulating thyroxine and triiodothyronine) include
- fatigue
- memory impairment
- slowed cognitive tempo
Moreover, thyroid levels (as measured by TSH-thyroid stimulating hormone), gradually decline with normal aging. One prominent hypothesis is that the magnitude of this decline in thyroid function is linked to aging-associated cognitive impairment.
The thyroid-cognition link makes a lot of intuitive sense. The brain is among the most metabolically active and energetically demanding organ systems in the body. Since thyroxine/triiodothyronine determine the metabolic set point, low-normal thyroid levels might impair cerebral metabolism and deleteriously impact cognition.
It has also been previously reported that thyroid hormones are potent and direct regulators of hippocampal neurogenesis in vivo. Triiodothyronine is often used as an augmentation strategy in the treatment of intractable mood disorders due to its antidepressant effects.
Downsides of Thyroxine Treatment
Synthroid (levothyroxine used to treat hypothyroidism) has been the all-time best selling drug in the United States for many years in a row.
America’s love affair with Synthroid has most likely suppressed our recognition that there are biological trade-offs with thyroid. Too much thyroid and the body is metabolically hyperactive, which induces oxidative stress. Since the generation of reactive oxygen species (ROS) is tightly coupled to metabolism, thyroid-induced increased metabolic rate concomitantly increases ROS.