Large Swedish study on T3 use and clinical outcome

[diogenes]

I have put here the title and abstract of a large Sweidish paper examining oucomes of T3 use. There is some evidence of bias here, especially where the lower mortality on T3 rather than T4 only is quoted but claimed to be an artefact (why, I wonder?). The get-out here is feeble. I think this article can be downloaded.

THYROID

Volume 31, Number 5, 2021ªMary Ann Liebert, Inc.

DOI: 10.1089/thy.2020.0388732

Liothyronine Use in Hypothyroidism and Its Effects on Cancer and Mortality

Tereza Planck, Fredric Hedberg, Jan Calissendorff, and Anton Nilsson

Background:The prescription of liothyronine (LT3) to treat hypothyroidism is increasing worldwide; however,the long-term safety of LT3 use has yet to be determined. Previous studies have suggested a possible associationbetween LT3 use and breast cancer. The aim of this study was to examine the effects of LT3 use on cancerincidence and mortality.Methods:Our sample included the full adult population of individuals living in Sweden with at least three purchases of thyroid hormone therapy between July 2005 and December 2017. Individual-level data on drug purchases were linked to registry data on cancer incidence and mortality. There were 575,461 individuals withat least three purchases, of which 11,147 had made at least three purchases of LT3, including combinations of levothyroxine (LT4) and LT3. Individuals were followed for a median follow-up time of 8.1 years. We appliedCox regression with a time-varying exposure variable, comparing LT3 users (individuals with at least three cumulative purchases of LT3) with LT4-only users (the rest).

Outcomes included breast cancer incidence, anycancer incidence, all-cause mortality, any cancer mortality, and breast cancer mortality. We adjusted for age,sex, previous thyroid cancer, previous other cancer, use of antithyroid preparations, use of sex hormones, anddose in multivariate analyses.

Results:Multivariate analyses produced a hazard ratio of 0.93 (95% confidence interval [0.75–1.15]) for breastcancer incidence (only females), 0.97 (0.87–1.08) for any cancer incidence, 0.69 (0.61–0.77) for all-cause mortality, 0.78 (0.62–0.98) for any cancer mortality, and 0.91 (0.50–1.66) for breast cancer mortality (only females).Conclusions:In this large, Swedish, long-term registry-based study, the use of LT3 did not lead to increasedbreast cancer incidence, any cancer incidence, all-cause mortality, any cancer mortality, or breast cancer mortalitycompared with LT4 use. Somewhat surprisingly, there was evidence of lower mortality in LT3 users in models adjusting for dose, potentially an artifact of underlying associations between dose and health

[LindaC]

Thank you, once more!

[Musicmonkey]

Thank you diogenes !

[knitwitty]

Many thanks for posting, as always, informative.

[SarahJane1471]

👍🙌

[jimh111]

The DOI reference seems invalid. I use doi.org/10.1089/thy.2020.0388 .

I'm writing up a topic on levothyroxine and cancer which includes some notes on this study.

The study is biased from the outset in that it's aim was to "examine the effects of LT3 use on cancer incidence and mortality". This allows them to make the valid conclusion "the use of LT3 did not lead to increased ... mortality compared with LT4 use". Had they used a neutral approach such as "the comparative safety of LT3 and LT4" the appropriate conclusion would have been that LT4 monotherapy carries additional risk as compared to LT3/LT4 combination thererapy.

It's a fair comment that there is a potential for bias in that doctors may be reluctant to prescribe LT3 to patients with cardiac problems, this irrational behaviour happens. On the other hand patients who need T3 are often severly hypothyroid and been left to rot for many years - a much greater bias.

Cancer mortality will not be subject to bias. The study shows a higher cancer risk with LT4 monotherapy, with even greater cancer mortality suggesting a harmful effect of LT4. Other studies do not find this association.

[shaws Administrator in reply to jimh111]

Switching from T4 to T3 calmed my heartbeat down to normal and saved the cardiologist inserting an implant in my heart 'to see what was going on during the night' when I had my worst palpitations.

[Hay2016 in reply to shaws]

Yep. Just adding 10mcg t3 meant my maximum exercising hr dropped from 210 😬to 170. Most heart palpitations gone which has to be better for health. X

[Hoxo]

An artifact! What have they got against T3?! How unscientific to dismiss results as an artifact when it goes against a scientific bias.

[shaws Administrator in reply to Hoxo]

Consider that no human being can function if they don't have T3 in their system. Brains and hearts have the most T3 receptor cells and we have T3 receptor cells all over our bodies.

thyroidpatients.ca/2019/12/...

[thyr01d]

Thanks Diogenes, very interesting and cheering.