Partly due to a specific question, I have decided to post this abstract despite misgivings. The very last thing I want to do is cause any panic. At the same time, the question was entirely reasonable and it is certainly not my role to censor access to research in any way.
The abstract claims that those women who take levothyroxine have a higher rate of breast cancer diagnosis. On the positive side, it is a large study.
On the negative side, it appears simply to associate the taking of levothyroxine with breast cancer diagnosis. It does not go further than making a suggestion that levothyroxine might cause breast cancer.
We have one immediately obvious issue, those who take levothyroxine will have been diagnosed hypothyroid (or, perhaps, have had a thyroidectomy). I think this is a major difference between the cases and the controls.
We can then launch into questions such as:
● Is it inappropriate dosing with levothyroxine rather than the substance itself? That is, we see so many who are on too little or too much.
● Or that so many will have gone through periods, sometimes of many years, in which they were hypothyroid before starting levothyroxine?
● Is it taking levothyroxine-only - hence the issue being lack of T3 rather than levothyroxine itself?
● Is it related to the autoimmune processes which are surely going on in many of those diagnosed hypothyroid?
● Is it the active ingredient, levothyroxine sodium, or the other constituents (excipients and breakdown/degradation products)? This could mean that the actual products available in Taiwan need to be looked at separately to those in, say, the UK.
● Although the paper does acknowledge length of time that levothyroxine has been taken (identifying the 2 month to 1 year group), it really is unclear how the duration of taking levothyroxine might affect anything. Yes - over-65 has a higher Odds ratio, but we have no indication as to how long they had been taking levothyroxine. If, for example, the over-65s had been taking for forty years, what rate would have been predicted?
Archives of Gynecology and Obstetrics
Levothyroxine use and the risk of breast cancer: a nation-wide population-based case–control study
Authors and affiliations
Chieh-Chen Wu 1 4
Ya-Yu Yu 2
Hsuan-Chia Yang 3 4
Phung Anh Nguyen 4 5
Tahmina Nasrin Poly 1
Md. Mohaimenul Islam 1 4
Usman Iqbal 4 7
Hafash Arshed Ali Khan 4
Yao-Chin Wang 1 8
Yung-Tzu Cheng 1 9
Yu-Chuan Li 1 4
Wen-Shan Jian 4 6
1.Graduate Institute of Biomedical Informatics, College of Medicine Science and TechnologyTaipei Medical UniversityTaipeiTaiwan
2.Tri-Service General Hospital, Songshan Branch Taipei Taiwan
3.College of Medicine Science and TechnologyTaipei Medical University Taipei Taiwan
4.International Center for Health Information Technology (ICHIT) Taipei Medical University Taipei Taiwan
5.Department of Population and Quantitative Health Sciences Case Western Reserve University Cleveland USA
6.School of Health Care Administration Taipei Medical University (TMU) Taipei Taiwan
7.Master’s Program in Global Health and Development Department, College of Public Health Taipei Medical University Taipei Taiwan
8.Department of Emergency Min-Sheng General Hospital Taoyuan Taiwan
9.Department of Laboratory Medicine Cathay General Hospital Taipei Taiwan
First Online: 30 June 2018
To investigate whether the use of levothyroxine was associated with breast cancer risk.
We conducted a population-based case–control study in Taiwan. Cases consisted of all patients who were aged 20 years and older, and had a first-time diagnosis of breast cancer for the period between 2001 and 2011. The controls were matched to the cases by age, sex, year, and month of diagnosis. Adjusted odd ratios (ORs) and 95% confidence intervals (CIs) were estimated by a conditional logistic regression.
We examined 65,491 breast cancer cases and 261,964 controls. We found that use of levothyroxine was associated with a significant increase in breast cancer risk (OR 1.24, 95% CI 1.15–1.33; P < 0.001). Compared with no use levothyroxine, the adjusted odd ratio was 1.22 (95% CI 1.11–1.35; P = 0.01) for the group having been prescribed levothyroxine 2 months to 1 year, and 1.26 (95% CI 1.12–1.41; P < 0.01) for the group with more than 1 year. When stratified by age, the adjusted odd ratio was 1.45 (95% CI 1.23–1.71; P < 0.01) for the patients aged 65 years or more and 1.19 (95% CI 1.09–1.29, P < 0.01) for the patients aged less than 65 years.
The results of the present study are the first to suggest that levothyroxine use increased the risk of breast cancer. However, a larger long-term prospective randomized-controlled trial specifically designed to assess the effect of levothyroxine use on the risk of developing breast cancer is needed.