Introduction

Photobiomodulation (PBM) – also known as low-level laser or light therapy – involves the non-invasive use of red and near-infrared light to modulate cellular function. PBM uses non-ionizing light (typically 600–1100 nm) delivered by lasers or LEDs at low power densities to stimulate biological responses without heating tissue pmc.ncbi.nlm.nih.gov . Originally developed for wound healing and pain relief, PBM has gained attention for its potential benefits in hormonal and endocrine health. Research in recent years has explored PBM’s effects on the thyroid gland and on reproductive hormones, with promising findings in conditions like autoimmune hypothyroidism, polycystic ovary syndrome (PCOS), menopause-related symptoms, and even infertility pmc.ncbi.nlm.nih.gov pubmed.ncbi.nlm.nih.gov . The appeal of PBM in these contexts lies in its ability to improve tissue function and reduce inflammation safely and painlessly. This review provides a concise overview of key studies on how PBM influences thyroid hormones and sex hormones, highlighting mechanisms of action and clinical applications in women’s health (perimenopause, menopause, endometriosis, fertility) and in male fertility. Human clinical studies are emphasized, but relevant animal research is included (noted clearly as animal studies) to illustrate underlying mechanisms or preliminary efficacy where human data are still emerging.

Mechanisms of Action

PBM exerts its effects at the cellular level primarily by enhancing mitochondrial function. Chromophores within mitochondria – particularly cytochrome c oxidase in the respiratory chain – absorb photons in the red/NIR spectrum pmc.ncbi.nlm.nih.gov. This photonic energy accelerates electron transport, leading to increased adenosine triphosphate (ATP) production, modulation of reactive oxygen species (ROS), and the release of nitric oxide (NO) from the enzyme’s catalytic center pmc.ncbi.nlm.nih.gov mcgill.ca . The immediate result is improved cellular energy metabolism and a transient, controlled increase in ROS that triggers redox-sensitive signaling pathways. PBM has been shown to activate transcription factors such as NF-κB, CREB, and others, which then induce gene expression changes that promote cell survival, proliferation, and anti-inflammatory profiles pmc.ncbi.nlm.nih.gov pmc.ncbi.nlm.nih.gov . A key outcome of these molecular events is reduced oxidative stress and inflammation in tissues. For example, PBM can induce anti-inflammatory cytokines (such as an increase in TGF-β in autoimmune thyroiditis models) while suppressing pro-inflammatory mediators, helping restore immune tolerance in affected tissues mdpi.com. PBM also improves microcirculation by dilating blood vessels (partly via NO release) and enhancing blood flow to treated areas mcgill.ca . In the context of endocrine organs, this means better oxygenation and nutrient delivery to glands like the thyroid or ovaries. Enhanced circulation, together with direct cellular stimulation, can improve gland function and hormone output mdpi.com . In summary, PBM’s mechanism involves mitochondrial photostimulation leading to higher ATP, moderated ROS/NO signaling, reduced inflammation, and improved tissue perfusion. These effects collectively set the stage for systemic hormonal regulation by rejuvenating the cells of endocrine glands and normalizing their signaling environment.

Effects on Thyroid Function

One of the most well-studied endocrine applications of PBM is for thyroid health, particularly hypothyroidism due to Hashimoto’s thyroiditis (autoimmune thyroid disease). Multiple clinical studies, including controlled trials, have found that PBM can significantly improve thyroid hormone levels and reduce the need for thyroid medication. In a landmark randomized placebo-controlled trial on 43 patients with Hashimoto’s hypothyroidism, Höfling et al. (2013) applied 10 sessions of 830 nm low-level laser to the thyroid region pubmed.ncbi.nlm.nih.gov

pubmed.ncbi.nlm.nih.gov . After 9 months, nearly half (47.8%) of the PBM-treated patients were able to maintain normal thyroid function without any levothyroxine replacement, whereas all placebo-treated patients still required full thyroid hormone doses pmc.ncbi.nlm.nih.gov . The PBM group had a dramatically lower thyroxine dose requirement (on average ~39 µg/day vs ~107 µg/day in controls, p <0.001) and showed a significant drop in thyroid peroxidase antibody (TPO-Ab) levels pubmed.ncbi.nlm.nih.gov . These results suggest that PBM can restore a degree of natural thyroid hormone production, likely by reducing autoimmune-driven gland damage. A larger 2020 clinical study (350 Hashimoto’s patients) by Ercetin et al. reported similarly striking outcomes: PBM treated patients showed a 70-fold greater increase in the T3/T4 hormone ratio compared to controls and required significantly lower doses of levothyroxine therapy pubmed.ncbi.nlm.nih.gov pubmed.ncbi.nlm.nih.gov . Over six months, PBM combined with standard supplements (selenium, vitamin D, etc.) led to higher T3, a normalized T3:T4 ratio, and a significant reduction in anti-TPO antibody titers, relative to supplement-only patients pubmed.ncbi.nlm.nih.gov . Another recent trial in 2023 confirmed that near-infrared PBM (820 nm, 6 sessions) plus supplements lowered TSH and thyroid antibody levels and induced weight loss in women with Hashimoto’s, whereas supplements alone did not – indicating improved thyroid gland output in the PBM group mdpi.com . Clinically, patients often report symptom improvements (fatigue, cold intolerance, etc.) alongside these biochemical gains

pubmed.ncbi.nlm.nih.gov. The mechanisms behind PBM’s thyroid benefits are actively investigated. PBM’s anti-inflammatory action appears critical: by dampening the lymphocytic attack on the thyroid (as evidenced by reduced autoantibodies and inflammatory markers), PBM allows the gland to recover function pubmed.ncbi.nlm.nih.gov . Ultrasound studies have even documented increased thyroid echogenicity after PBM, consistent with reduced inflammation and partial regeneration of thyroid tissue structure pmc.ncbi.nlm.nih.gov . Notably, PBM may promote thyroid follicular cell regeneration. Animal studies show that PBM can stimulate thyroid tissue repair: in rats with thyroid injury, PBM accelerated follicle regeneration and revascularization of the gland mdpi.com . In healthy animal models, PBM has been found to improve thyroid microcirculation and actually elevate serum T3 and T4 levels mdpi.com , supporting the idea that PBM directly enhances thyroid hormone release (these animal findings provide a mechanistic complement but human relevance is being confirmed). Importantly, long-term follow-up indicates that PBM effects can be durable and that the treatment is safe. A 6-year follow-up of patients from the Höfling trial found that those who had received PBM maintained better thyroid function and did not show any increase in thyroid nodule formation or cancer compared to controls

pmc.ncbi.nlm.nih.gov mdpi.com . In summary, PBM has demonstrated the ability to restore thyroid hormone balance in autoimmune hypothyroidism – increasing T3 and T4, normalizing TSH, and even reducing autoimmune antibody levels – thereby lessening or sometimes eliminating the need for lifelong hormone medication pubmed.ncbi.nlm.nih.gov

pubmed.ncbi.nlm.nih.gov . This represents a remarkable shift in managing a condition traditionally thought to be irreversible, and it underscores PBM’s potential as a regenerative therapy for the thyroid gland.

PBM and Sex Hormones

Female Sex Hormones (Estrogen and Progesterone). Early research suggests PBM can positively influence female hormonal production and ovarian function. Direct human data are still limited, but animal studies provide proof-of-concept that PBM may rejuvenate ovarian tissue and modulate the estrogen–progesterone axis. In an animal study of natural ovarian aging (in middle-aged mice), He et al. (2024) demonstrated that PBM (650 nm laser, applied over 2 months) effectively restored sex hormone levels in aged female mice to more youthful profiles

pubmed.ncbi.nlm.nih.gov. Treated mice showed higher ovarian estrogen output and improved progesterone levels, corresponding with an increased number of ovarian follicles (including primordial and growing follicles) compared to untreated aging mice pubmed.ncbi.nlm.nih.gov

. PBM also enhanced ovarian blood vessel formation and reduced apoptosis in ovarian cells, indicating a healthier ovarian environment for hormone production. These findings imply that PBM could delay or reverse some aspects of ovarian aging, potentially easing the hormonal fluctuations of perimenopause. Consistent with this, a study in a polycystic ovary syndrome (PCOS) rat model (PCOS is characterized by anovulation and hormonal imbalance) reported that ovarian-targeted PBM helped normalize sex hormones and ovarian cycles pubmed.ncbi.nlm.nih.gov . In that 2019 study by Alves et al., PCOS-induced rats were treated with low-level laser (3 times/week); PBM-treated rats had higher progesterone

levels and lower luteinizing hormone (LH) compared to untreated PCOS rats pubmed.ncbi.nlm.nih.gov . Moreover, PBM increased the number of healthy ovarian follicles and corpora lutea while reducing the number of cystic, atretic follicles pubmed.ncbi.nlm.nih.gov. This indicates a restoration of ovulatory cycles – the increase in progesterone and corpus lutea suggests that previously anovulatory (PCOS) rats resumed normal ovulation after PBM therapy. Notably, PBM also slightly raised testosterone levels in these female rats pubmed.ncbi.nlm.nih.gov, which might reflect normalization of ovarian theca cell function (in PCOS models with estradiol-induced ovarian suppression, baseline androgens can be low; PBM bringing testosterone into a normal range may be part of reestablishing hormonal balance). While human trials in women with PCOS or ovarian insufficiency are forthcoming, these animal results highlight PBM’s potential to modulate ovarian endocrine activity – increasing estrogen/progesterone output through improved folliculogenesis and ovulation. Indirect evidence in humans comes from case series where women with irregular cycles reported improved menstrual regularity after a course of PBM, coinciding with better ovulatory function

pubmed.ncbi.nlm.nih.gov pubmed.ncbi.nlm.nih.gov (though controlled data are needed). Taken together, PBM appears to act as a pro-gonadal stimulus in females, supporting estrogen/progesterone production by healing or activating ovarian tissue.

Male Sex Hormones (Testosterone) and Testicular Function. There is intriguing evidence that PBM can also enhance testosterone production and male fertility parameters, primarily from animal studies. The testes, like the thyroid, are metabolically active organs that can respond to photobiomodulation. In a pioneering rat study, researchers applied low-level 904 nm laser light to rat testes and observed a significant increase in serum testosterone levels compared to controls pmc.ncbi.nlm.nih.gov . Importantly, this increase in testosterone was achieved without any histological damage to testicular tissue, suggesting that PBM stimulated Leydig cells (the testosterone-producing cells) in a physiological manner. Additional animal experiments confirm that PBM improves testicular function, especially under stress conditions. For example, PBM has been tested in models of heat-induced testicular injury – a scenario relevant to male fertility since excessive scrotal heat can impair spermatogenesis. In a 2023 study, adult male mice exposed to scrotal hyperthermia (to induce transient infertility) were treated with PBM to the testes. The PBM-treated mice showed marked recovery of spermatogenesis : sperm counts and motility, which had plummeted due to heat, rebounded significantly in the PBM group versus untreated controls pmc.ncbi.nlm.nih.gov pmc.ncbi.nlm.nih.gov . PBM also countered oxidative stress in the testes – levels of reactive oxygen species were lower and glutathione (an antioxidant) higher in treated mice, indicating a restoration of the redox environment conducive to sperm production

pmc.ncbi.nlm.nih.gov . Crucially, even though the heat injury had drastically reduced serum testosterone, PBM therapy helped partially restore testosterone levels toward normal ranges (from 0.8 ng/mL in hyperthermia-only mice up to ~1.1 ng/mL after PBM, vs 1.36 ng/mL in healthy controls) pmc.ncbi.nlm.nih.gov pmc.ncbi.nlm.nih.gov . It also increased the number of Leydig cells that had been depleted by heat pmc.ncbi.nlm.nih.gov pmc.ncbi.nlm.nih.gov. These results demonstrate that PBM supports both the somatic and endocrine functions of the testes: improving sperm output and boosting androgen (testosterone) production in animal models. Though direct clinical trials in men are limited, there are anecdotal and preliminary reports of PBM (using red light therapy devices) improving testosterone levels in men with low T, and some fertility clinics have begun exploring red light therapy to improve sperm parameters. In vitro studies on human sperm have shown that PBM can increase sperm motility and viability, which could translate into better male fertility outcomes (discussed more under “Fertility” below)

pmc.ncbi.nlm.nih.gov onlinelibrary.wiley.com . Overall, the evidence to date – largely from animal research – indicates PBM can positively influence sex hormone production: enhancing estrogen/progesterone output in females and testosterone in males, by energizing the ovaries and testes and reducing local inflammatory or oxidative stress that impairs endocrine function. These findings set the stage for clinical applications in hormone-related disorders.

Clinical Applications in Women’s Health