Thyroid, iodine, and breast health

I wrote a research paper looking the relationships between thyroid function, breast pathologies, and the role of iodine supplementation. This blog is a summary of what I learned. References are listed at the end and can be found on PubMed.

ThyroidThyroid and breast health

There is an observed correlation between thyroid dysfunction and breast cancer (1,2,3), particularly hypothyroidism (3,4). Decreased function of the thyroid gland and rising TSH are also associated with a doubling in the risk of development of fibrocystic breast disease (5).

Iodine and breast health

Both breast tissue and thyroid tissue concentrate iodine (6) and deficiency of iodine causes “atypical tissue and physiologic changes in both” (7). One researcher noted that “geographic differences in the rates of breast, endometrial, and ovarian cancer appear to be inversely correlated with dietary iodine intake” (8). For example, in Japan seaweed (high in both iodine and selenium) is a major part of the diet and may play a role in the low incidence of both benign and malignant breast disease in that country (9).

A study on rats demonstrated a reduction in breast cancer incidence with iodine treatment (10). With regard to fibrocystic breast disease, randomized, double-blind, placebo-controlled, multicenter clinical trials (the gold-standard of conventional medicine!) have demonstrated that supplementation with iodide/iodine significantly reduced breast pain, tenderness, and nodularity (11,12).

So how does this work? One theory is that iodine deficiency leads to a state of excess estrogen (13), increasing the risk of a whole host of cancers. Iodine is also believed to suppress tumour growth (14), induce tumour cell death (15), and regulate genes that influence hormone metabolism, cell cycle, growth, and differentiation (16).

Naturopathic gynaecologySo what?

Why is this useful information? Everything in the body is connected, particularly the endocrine system. So, when something is going wrong in one area, like thyroid function, there are likely also going to be issues either immediately or down the road with another area, particularly reproductive organs. The causal mechanisms are not clear yet, but it is prudent to assess breast health when patients present with thyroid dysfunction and vice versa. There is evidence to suggest that iodine is a helpful nutritional treatment for fibrocystic breast disease and it may also be useful in reproductive cancers. Molecular iodine, rather than iodide, was found to be best and have the least adverse effects on the thyroid (12).

There is lots of other interesting information that I was not able to get to, such as a deeper look at the interaction of sex hormones with thyroid function and other influences on hormonal balance, such as sleep. Maybe one day I’ll have time to do more research and I’ll post an update!


  1. Turken O, NarIn Y, DemIrbas S, Onde ME, Sayan O, KandemIr EG, YaylacI M, Ozturk A. Breast cancer in association with thyroid disorders. Breast Cancer Res. 2003;5(5):R110-3. Epub 2003 Jun 5. PMID: 129270402.
  2. Saraiva PP, Figueiredo NB, Padovani CR, Brentani MM, Nogueira CR. Profile of thyroid hormones in breast cancer patients. Braz J Med Biol Res. 2005 May;38(5):761-5. Epub 2005 May 25. PMID: 15917958
  3. Giani C, Fierabracci P, Bonacci R, Gigliotti A, Campani D, De Negri F, Cecchetti D, Martino E, Pinchera A. Relationship between breast cancer and thyroid disease: relevance of autoimmune thyroid disorders in breast malignancy. J Clin Endocrinol Metab. 1996 Mar;81(3):990-4.
  4. Kuijpens JL, Nyklíctek I, Louwman MW, Weetman TA, Pop VJ, Coebergh JW. Hypothyroidism might be related to breast cancer in post-menopausal women. Thyroid. 2005 Nov;15(11):1253-9. PMID: 16356089
  5. Mardaleishvili KG, Nemsadze GG, Metreveli DS, Roinishvili TL. [About correlation of dysfunction of the thyroid gland with fibrocystic diseases in women] Georgian Med News. 2006 Nov;(140):30-2. Russian.
  6. Patrick L. Iodine: deficiency and therapeutic considerations. Altern Med Rev. 2008 Jun;13(2):116-27. Review. PMID: 18590348
  7. Eskin BA, Grotkowski CE, Connolly CP, Ghent WR. Different tissue responses for iodine and iodide in rat thyroid and mammary glands. Biol Trace Elem Res. 1995 Jul;49(1):9-19. PMID: 7577324
  8. Stadel BV. Dietary iodine and risk of breast, endometrial, and ovarian cancer. Lancet. 1976 Apr 24;1(7965):890-1. PMID: 58152
  9. Cann SA, van Netten JP, van Netten C. Hypothesis: iodine, selenium and the development of breast cancer. Cancer Causes Control. 2000 Feb;11(2):121-7. Review. PMID: 10710195
  10. García-Solís P, Alfaro Y, Anguiano B, Delgado G, Guzman RC, Nandi S, Díaz-Muñoz M, Vázquez-Martínez O, Aceves C. Inhibition of N-methyl-N-nitrosourea-induced mammary carcinogenesis by molecular iodine (I2) but not by iodide (I-) treatment Evidence that I2 prevents cancer promotion. Mol Cell Endocrinol. 2005 May 31;236(1-2):49-57. Epub 2005 Apr 13. PMID: 15922087
  11. Kessler JH. The effect of supraphysiologic levels of iodine on patients with cyclic mastalgia. Breast J. 2004 Jul-Aug;10(4):328-36. PMID: 15239792
  12. Ghent WR, Eskin BA, Low DA, Hill LP. Iodine replacement in fibrocystic disease of the breast. Can J Surg. 1993 Oct;36(5):453-60. PMID: 8221402
  13. Stadel BV. Dietary iodine and risk of breast, endometrial, and ovarian cancer. Lancet. 1976 Apr 24;1(7965):890-1. PMID: 58152
  14. Funahashi H, Imai T, Tanaka Y, Tobinaga J, Wada M, Morita T, Yamada F, Tsukamura K, Oiwa M, Kikumori T, Narita T, Takagi H. Suppressive effect of iodine on DMBA-induced breast tumor growth in the rat. J Surg Oncol. 1996 Mar;61(3):209-13. PMID: 8637209
  15. Shrivastava A, Tiwari M, Sinha RA, Kumar A, Balapure AK, Bajpai VK, Sharma R, Mitra K, Tandon A, Godbole MM. Molecular iodine induces caspase-independent apoptosis in human breast carcinoma cells involving the mitochondria-mediated pathway. J Biol Chem. 2006 Jul 14;281(28):19762-71. Epub 2006 May 5. PMID: 16679319
  16. Stoddard FR 2nd, Brooks AD, Eskin BA, Johannes GJ. Iodine alters gene expression in the MCF7 breast cancer cell line: evidence for an anti-estrogen effect of iodine. Int J Med Sci. 2008 Jul 8;5(4):189-96. PMID: 18645607
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