Light therapy helps thyroid function: it assists inreversing hypothyroidism by modifying gonadotropin hormone release (32). Gonadotropins are hormones that, in women, stimulate the release of follicle stimulating hormones (FSH) and luteinizing hormones (LH) from the pituitary gland. In the female body, LH stimulates the production of testosterone, which is then converted to estrogen. FSH stimulates the egg follicles to mature, these follicles will eventually produce progesterone and small amounts of estrogen. Human gonadotropin(hCG) hormones are weak stimulators of thyrotropin or TSH. Human gonadotropin hormones stimulate a substance called adenosine monophosphate or cAMP (89) that then produces more thyroid stimulating hormone. In both humans and animals hCG has been shown in studies to stimulated thyroid activity. For instance, when people have overly high levels of hCG they are often diagnosed with hyperthyroidism (89).
This link between sex hormones and thyroid functioning in women should not be overlooked. Women are up to 20 times more likely to develop Hashimoto’s thyroiditis. This condition is linked to disruptions in sex hormones. Progesterone deficiency, often experienced in the menopausal transition, is interrelated with estrogen dominance (too much estrogen in comparison to the amount of progesterone). This condition is linked to thyroid problems. Estrogen increases the production of a protein made in the liver that inactivates thyroid hormone, called thyroid binding globulin or TBH (90). The end result of such an imbalance is a slow down of the metabolism and increased weight and body fat. Conversely, progesterone improves thyroid hormone activity by lowering the production of thyroid binding globulin. When thyroid hormones are active, the body’s metabolism is stimulated, energy levels are improved, more fat is burned, and weight is better managed (91). And, if estrogen dominance goes unchecked, hypothyroidism can be the result. There are several shared symptoms of estrogen dominance and hypothyroidism (fatigue, headache, hair loss, reduced sex drive, and weight problems). Estrogen dominance is also considered a symptom of thyroid insufficiencies (92). So, these two conditions, which overlap, can be mistaken for one another (95).
Hypothyroidism is associated with depression in research where other factors, like economics, race, age, and education levels, have been ruled out (93). When progesterone drops and estrogen goes up or is imbalanced (normal estrogen, but not as much or no progesterone) thyroid binding globulin (TBG) goes up. In response to elevated TBG, thyroid hormone T3 production is slowed. So, it becomes less available to cells, disrupting their functioning. This is one way in which progesterone deficiency disrupts limbic system functioning. There are thyroid hormone receptors on brain cells (102), and if the brain is deprived of thyroid hormones mood states and cognition can become compromised (103), especially in the limbic system (104). The limbic system is an area of the brain that includes the amygdala, hypothalamus, thalamus, hippocampus and other structures. These structures are involved in regulating mood, as well as motivation, learning, and memory. Making things worse, when T3 goes down, production of neurotransmitters serotonin and norepinephrine also drops. As these neurochemicals are involved in mood and though process, the result can be low mood, depression, and a lack of interest in pleasure, called anhedonia (94).
Keep in mind that thyroid disruptions can also impact sex hormones. Thyroid hormone T3 can galvanize the release of progesterone from luteal cells (100). Progesterone production mainly takes place during or slightly after ovulation. It happens in the ovaries, specifically in the corpus luteum, which is a temporary structure and the remains of an ovarian follicle, after it has released a mature ovum or egg. So, if women have trouble ovulating, they will have trouble producing enough progesterone. This is another reason that the menopausal transition is associated with thyroid problems.
Thyroid problems are also connected to the inflammatory response and the oxidative stress underlying it. Thyroid hormones are meant to play a protective role regarding damage due to inflammation and oxidative stress. But, if the thyroid is not functioning properly, this can’t happen. So, hypothyroidism can amplify oxidative stress, which can magnify inflammation, which can then suppress sex hormone production, causing yet more thyroid issues (101) and problems producing neurochemicals.
Melatonin, which is improved with light therapy, also impacts thyroid health. Both melatonin and thyrotropin releasing hormone or TRH are produced by the pineal gland. TRH stimulates the production of thyroid stimulating hormone or TSH. TSH then galvanizes the production of thyroid hormone. So, melatonin can directly stimulate TSH production (87). In a study of women lacking proper estrogen/progesterone production, those who took 3mg of melatonin for three to six months were shown to increase thyroid hormone levels (88).
Light therapy improves vitamin D production. Vitamin D deficiency is associated with thyroid disease (96). It one study, 72% of subjects with autoimmune thyroid disease had D deficiency (97). In another study, on Hashimoto’s thyroiditis, 85% of subjects had low levels of D, and high levels of anti-thyroid antibodies (98). And, it follows that vitamin D can help to treat thyroid problems. In one study, those patients who received 1,200 to 4,000 international units or IUs of vitamin D per day over a period of four months showed lower levels of anti-thyroid antibodies at the end of the treatment (98). Keep in mind that this is above the recommended daily allowance of 600 IUs. Regarding thyroid stimulating hormone specifically, in a study of hypothyroidism and vitamin D, those who took supplements for 12 weeks showed improved levels of thyroid stimulating hormone. Keep in mind that there was no change to thyroid hormones T3, thyroxin, T4, or triiodothyronine (99).
Depression is connected to sex hormone dysregulation. In the female body, LH stimulates the production of testosterone, which is then converted to estrogen. FSH stimulates the egg follicles to mature, these follicles will eventually produce progesterone and small amounts of estrogen. Female sex hormones are neuromodulators, they play a significant role in controlling the dopamine neurons, called nigrostriatal, found in the area of the brain linked to cognitive processes, called the Substantia nigra. In animal studies (female rats and primates) subjects who had their ovaries removed experienced a 30%, or higher, loss of dopamine cells (35). This is very important as neurotransmitter dopamine is produced in these cells. So, sex hormones, by way of the Substantia nigra, are central to cognition, emotion, and movement. The neurons in the Substantia nigra travel to the frontal lobes (attention and executive function) and the area of the brain associated with motor control. If there is a death of neurons in the Substantia nigra, cognitive problems, and loss of motor control, will result (36). Further, estrogen also intensifies the effects of norepinephrine (adrenaline in the brain) and serotonin. It does this partly by decreasing the monoamine oxidase (MAO) activity in the central nervous system (CNS). MAO breaks down serotonin and norepinephrine, so estrogen slows this process down, leaving the neurotransmitters active longer. This may explain the antidepressant like effect of melatonin (if given/taken at the right time of day) in perimenopausal and menopausal women (32). In a human study of natural morning light, it was found that post menopausal women who had less morning window covering and more morning light experienced less depressed mood (37).
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32 Zimmermann, R.C., & Olcese, J.M., (2007). Melatonin, in Treatment of the Postmenopausal Woman, Basic and Clinical Aspects, (third ed), 2007. Academic Press. Elsevier Inc., Amsterdam, Netherlands. DOI: 10.1016/B978-0-12-369443-0.x5000-5. Accessed at: https://www.sciencedirect.com/tompics/neuroscience/melatonin.
35 Leranth, C., Roth, R.H., Elsworth, J.D., Naftolin, F., Horvath, T.L., & Redmond, D. E., (2000). Estrogen is essential for maintaining nigrostriatal dopamine neurons in primates: implications for Parkinson’s disease and memory. Journal of Neuroscience 20:8604-8609.
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