Introduction
Soy foods have had an important dietary role in Asian countries for thousands of years ago, and due to the result of their suggested health benefits, in recent years they have become increasingly popular in Western countries as well. Nevertheless, there are some concerns that soy can have a negative effect on thyroid function and can alter the levels of thyroid hormones.
Thyroid is an important endocrine gland that makes and releases certain hormones. Its main function is to control your metabolism which means how your body uses energy. (1)
But sometimes, your thyroid doesn’t work properly, and this condition is treatable.
There is some concern that soy intake may interfere with thyroid function, possibly inducing hypothyroidism (low thyroid function).(5) In this review we have discussed the link between soy or soy product consumption and thyroid function via the measurement of thyroid hormone levels.
Thyroid gland
Thyroid is a small, butterfly-shaped gland located at the front of your neck under your skin. It’s a part of your endocrine system and controls many of your body’s important functions by producing and releasing (secreting) certain hormones. (1)
Its main role is to control the speed of your metabolism (metabolic rate), which is the process of how your body transforms your food consumption into energy. All of the cells in your body need energy to function. (1)
Your thyroid gland and its hormones affect almost every organ system of your body, including: Your cardiovascular system, your nervous system, your digestive system, your reproductive system. (1)
Summary
The thyroid is a large gland that regulates growth and controls the rate at which your body expends energy. (3)
Impact of flavonoids on thyroid hormones
High intake of soy products may suppress thyroid function in some people and contribute to hypothyroidism which is a condition characterized by low production of thyroid hormones. (2)
Goitrogens are substances that interfere with iodine uptake in the thyroid gland. Soy is a goitrogenic food that prevents proper thyroid hormone production and causes the thyroid gland to enlarge. It may also impact the thyroid by other mechanisms such as, preventing the action of thyroid hormones throughout the body and reducing absorption of thyroid medications from the intestines. (4)
Isoflavones are the major flavonoids found in legumes, particularly soybeans. Soy is known to produce estrogenic isoflavones. Animal and human studies indicate that the isoflavones found in soybeans may suppress the formation of thyroid hormones. There is a possibility that widely consumed soy products may cause harm in the human population by either or both estrogenic and goitrogenic activities is of concern. (6)
A study observed in 37 Japanese healthy adults, whether soybeans would suppress thyroid function, it was shown that eating 1 ounce (30 grams) of soybeans every day for 3 months caused symptoms related to suppressed thyroid function. The symptoms such as discomfort, sleepiness, constipation, and thyroid enlargement. The findings of the study suggested that excessive soybean ingestion for a certain duration might suppress thyroid function and cause goiters in healthy people, especially elderly subjects. All of the symptoms disappeared after the study ended. (7)
Summary
Some animal studies support a hypothyroidic effect of soy. (8) This could be due to the presence of phytoestrogens and goitrogens in soy. Goitrogens are substances that interfere with iodine uptake in the thyroid gland.
Effect of Soy on TSH levels
A systematic review and meta-analysis was undertaken on all randomized controlled trials of studies including soy as an intervention and where free triiodothyronine (fT3), free thyroxine (fT4) and thyroid stimulating hormone (TSH) was measured. (9)
Soy supplementation has no effect on the thyroid hormones and only very modestly raises TSH levels, the clinical significance, if any, of the rise in TSH is unclear. (9)
In a review, a long-term randomized controlled trial (RCT) have been published, which followed 403 menopausal women for two years, reported that 80 and 120 mg of soy isoflavones had no significant effect on TSH and free T4. (10) The effect on free T4 was nearly significant (-8%, from 1.2 to 1.1 ng/dL), but free T4 remained well within normal range. Other parameters of thyroid status, such as free T3, were not reported.
Summary
The evidence regarding the impact of soy on thyroid health is limited, also studies do not show any link between soy and TSH or thyroid function.
Effect of Soy on Thyroid Function
A comprehensive review in 2006, of 14 studies in humans does not report any significant adverse effects of soy foods and Isoflavones consumption on thyroid activity in healthy adults (with normal thyroid function and sufficient iodine intake), whereas people with compromised thyroid function or insufficient iodine intake, also infants born with thyroid hormone deficiency may be considered at greater risk of developing hypothyroidism if they consume soy foods. (11, 12)
Since the publication of this review, two more long term randomized controlled trials (RCT’s) have been published. It also reported that daily supplementation with 54 mg of genistein had no significant effect on thyroid-antibody concentrations, thyroid-hormone concentrations, or thyroid-receptor activity. (11)
The human evidence suggests that soy foods do not have any significant impact on thyroid health. Neither the review nor the two more recent RCTs had thyroid activity as their primary outcome. (11)
The authors of the 2016 study indicate that soy is likely to be safe. But consuming excessive amounts may create health risks in some people, such as those with undiagnosed hypothyroidism. However, there’s no evidence that people who have hypothyroidism should avoid soy completely.
Summary
In short, regular consumption of soy products or isoflavone supplements may lead to hypothyroidism in sensitive individuals, especially those who have an underactive thyroid gland. However, there’s no evidence that people who have hypothyroidism should avoid soy completely. In conclusion, the human evidence suggests that soy does not have any pragmatically meaningful effects on thyroid health.
Conclusion
The human evidence suggests that soy foods do not have any significant impact on thyroid health. Neither the review nor the two more recent RCTs had thyroid activity as their primary outcome. There is no effect of Soy supplementation on the thyroid hormones and very modestly raises TSH levels, the clinical significance, if any. The rise in TSH level is unclear. The evidence regarding the impact of soy on thyroid health is limited, also studies do not show any link between soy and TSH or thyroid function. Conclusively soy does not appear to affect thyroid activity in humans.
References:-
- Thyroid: What It Is, Function & Problems (clevelandclinic.org)
- Unawareness of the effects of soy intake on the management of congenital hypothyroidism – PubMed (nih.gov)
- Soybeans 101: Nutrition Facts and Health Effects (healthline.com)
- Impact of flavonoids on thyroid function – ScienceDirect
- Soy: Types, benefits, and nutrition (medicalnewstoday.com)
- Goitrogenic and estrogenic activity of soy isoflavones – PubMed (nih.gov)
- [The effects on the thyroid gland of soybeans administered experimentally in healthy subjects] – PubMed (nih.gov)
- Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo – PubMed (nih.gov)
- Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function | Scientific Reports (nature.com)
- Clinical outcomes of a 2-y soy isoflavone supplementation in menopausal women – PubMed (nih.gov)
- Is soy good or bad for you? – Examine
- Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature – PMC (nih.gov)
