Green Tea & Your Thyroid: Are the T4 & T3 Reducing Effects of 250mg (HED) Green Tea Catechins Reason For Concern?

Is there a devilish thyroid hormone eating dragon hiding out in your beloved green tea?
There is hardly a day without great, yet mostly only promising, and rarely really relevant news about green tea. With it's anti-cancer, anti-diabetes, anti-everything-that-ails our society effects it bids fair to be the elixir of life. This life, and this is the result of a recently published study, SuppVersity reader Paolo had brought to my attention a couple of days ago, could however end up being one with slightly or even profoundly compromised thyroid function. Now, before you freak out because you have been drinking green tea for the past couple of years, do me a favor, mind the conditional in the previous sentence and read the rest of today's Easterly SuppVersity article, which is not an early April fool hoax.

"You must be kiddin' green tea helps weight loss, so how can it reduce thyroid function?"

It may sound hilarious that something that is touted as a fat burner and weight loss adjuvant with tons of scientific backup is supposed to have the nasty ability of inducing (probably transient) hypothyrodism. The data Amar K. Chandra and Neela De, University of Calcutta and the University College of Science & Technology present in their most recent paper is yet unambigous: The hailed green tea catechin and flavonoids possess "potent antithyroid activity as evidenced from in vivo and in vitro studies" (Chandra. 2013)

The 20% reduction in testosterone in response to 5 cups/day of green tea (HED) researchers observed in a 2011 rodent study shows that the thyroid is not the only organ that does not like green tea catechins (learn more),
Despite the fact that this is probably news to most of you, the results of Chandra's and De's most recent experiment do actually line up pretty nicely with previous data by the same researchers (2010), as well as the goitrogenic (T3+T4 low + TSH high => abnormal growth of the thyroid) effects the daily administration of green tea extracts had on the lab animal of Y Sakamoto and his colleagues from the Tokyo Metropolitan Research Laboratory of Public Health in Japan (Sakamoto. 2001).

Moroever, their data supports the general notion that the hailed green tea flavon-3-ols (flavenols) can mess with all sort of enzymatic conversion processes in the mammalian body - including the aromatization of testosterone to estrogen (Sato. 2002).

It's not just the inhibition of iodine uptake that's the problem here. It's its release and conversion.

By acting directly on the enzymatic activities of thyroid peroxidase and 5'-deiodinase I it effectively blocks the generation of T4, by inhibiting the release of iodine (thyroid peroxidase) and subsequent conversion of the latter to thyroxine (T4), the major thyroid hormone in the mammalian body and precursor to the "metabolically active" triiodothyronine (T3).
Figure 1: Weight gain, thyroid cell morphology, enzyme activity and thyroid hormone levels after 30 days on different amounts of green tea catechins; all data expressed relative to untreated control (Chandra. 2013)
As the data in figure 1 goes to show you, the administration of pure green tea catechins at dosages of 10, 20, and 30 mg/kg body weight (intraperitoneal ~ orally, but in a way that the animal cannot puke it up) for two, respectively four weeks (15 vs. 30 days), led to statistically highly significant and dose-dependent decreases of the activity of the aforementioned enzymes and concomitant increases in Na+, K+ ATPase activity, as well as substantial decrease in serum T3 and T4 levels that went hand in hand with elevations of the thyroid stimulating hormone TSH. In view of these clearly goitrogenic effects, it's therefore not surprising that ...
"[h]istological examinations of the thyroid gland revealed marked hypertrophy and/or hyper-plasia of the thyroid follicles with depleted colloid content." (Chandra. 2013)
What we do yet  have to keep in mind is that rodents are in general more sensitive to goitrogenic agents (Döhler. 1979; Capen. 1995) so that the conclusion that dosages as low as 3-4mg/kg of a highly concentrated green tea extract could lead to fulminant reductions in thyroid function or even full blown goitre, is clearly unwarranted.
"Nutritional thyroid medication" - Sirloin of beef in smoked butter. Those plus tons of veggies and a controlled amount of fruits were the "magical" cure to low thyroid function in kids in a 2012 study shows, you may remember from the SuppVersity news (refresh your memory)
Thyroid function and body composition: Just in case you have been missing Tuesday's Facebook news on the correlation of T4 and T3 levels with total body mass, body fat, waist cirumference and more here is a brief reminder. In the 100 euthyroid men, Min Kyong Moon and his colleagues from the Department of Internal Medicine at the Seoul National University College of Medicine in Korea observed that free T3 is inversely correlated with body mass index, LDL, intramuscular fat area and the total amount of liver fat (Moon. 2013). The levels of free T4, on the other hand, also showed inverse correlations with the waist circumference and total body weight of the Koreans. In addition, both, fT3 & fT4 were negatively associated with pericardial fat.
Chandra and De are yet nevertheless right, when they point out that "human tea drinkers", and even more people who consume large amounts of the commercially available green tea extracts that have been used in the study at hand, are likely to be be "at risk", as well (Chandra. 2013). How real this risk eventually is, and which amount of green tea extracts would be necessary to induce similarly negative effects in humans would yet (as so often) require further investigation.

Bottom line: I am no friend of the notion that the consumption of large amounts isolated extracts of whatever purported health-elixir will have nothing but benefits. The current evidence is yet far from being conclusive enough to give up on your one, two or three cups of green tea per day.

Though color may matter in terms of the thyroid effects, the most important thing for anyone trying to keep his waist tight appears to be that he/she drinks tea, whether it is green, black, white or well... pu-erh (learn more ;-)
And I am not saying that, because I am afraid of going back on a previous advice, but simply because thyroid hormone abnormalities have yet never been an issue in any of the myriad of studies on the beneficial effects of regular green tea consumption, of which Crespy et al. who don't simply ignore potential downsides of green tea extracts in their 2004 review, explicitly state that it would "even [in] a very high dietary amount [...] be unlikely to cause" (Crespy. 2004) these types of effects. And if you want to make 100% sure, just exchange one or two of the cups of green tea with its fermented black cousin which has a way lower goitrogenic potential (Chandra. 2011), and still shares many of he beneficial effects of its "raw" relative (cf. Leung. 2001; Gupta. 2002; Gardner. 2007, ...)

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