# Do Vegetable Oils Harm Your Thyroid? The Truth About Polyunsaturated Fats

By Dr. Benedicte Lerche · Metabolic Health · Published 2026-04-09

> We are told to replace saturated fats with vegetable oils. But research shows that polyunsaturated fats promote cell aging, inflammation, obesity, diabetes, hypothyroidism, and hormonal imbalances. Here is the truth about these common cooking oils.

We are told to use polyunsaturated oils instead of saturated fats. But research tells us that consuming polyunsaturated fats leads to cell aging, inflammation, obesity, diabetes, hypothyroidism, and hormonal imbalances. This post tells you the truth about polyunsaturated fats — and why the oils sitting in most kitchen cupboards may be quietly damaging your health.

Polyunsaturated fats are liquid oils such as corn, canola (rapeseed), soy, safflower, sunflower, flaxseed (linseed), sesame, and peanut oil. Saturated fats, in contrast, are found in coconut oil and butter. Health authorities and the media tell us that eating polyunsaturated fats in place of saturated fats protects against heart disease. However, according to several researchers — including [Dr. Ray Peat](/blog/who-is-dr-ray-peat) — eating saturated fats has no negative effect on heart and arterial health. It is the polyunsaturated fats that are unhealthy and damaging.

According to many scientists and nutritionists, including Dr. Ray Peat, the consumption of polyunsaturated fats is linked to obesity, cell aging, inflammation, and hormonal imbalances including hypothyroidism and insulin resistance (diabetes).

## The Cholesterol Myth

In the 1960s and 1970s, people were eating a diet high in saturated fats — including butter and coconut oil — and they were generally lean and healthy. Then misleading research linked saturated fats to heart disease, which changed dietary recommendations toward polyunsaturated fats.

It is true that eating saturated fats raises cholesterol, but there is no solid scientific evidence connecting cholesterol to heart disease. According to research, heart disease and arteriosclerosis are caused by inflammation. Cholesterol is present in injured arteries as a protective molecule — it is not the cause of arteriosclerosis.

The notion that cholesterol causes heart disease has been repeated so often that people accept it as fact. This cholesterol myth has led to dietary guidelines that promote polyunsaturated fats as "heart-healthy" while demonising saturated fats. Prepared foods that once contained saturated fats now contain polyunsaturated fats — meaning the modern Western diet is saturated with these unstable oils, with devastating health consequences for the population.

In my [courses](/courses), I cover cholesterol and heart disease in depth, examining the actual evidence and what it means for your thyroid and metabolic health.

## Polyunsaturated Fats Are Chemically Unstable

The structural difference between unsaturated and saturated fats gives them very different biological properties.

In saturated fats, the carbon atoms are connected by single bonds — which are very stable and non-reactive. In unsaturated fats, some carbon atoms are connected by double bonds, which introduces an open and unstable structure into the fat molecule. Monounsaturated fats have one double bond, while polyunsaturated fats contain more than two double bonds. These multiple double bonds make polyunsaturated fats highly unstable and prone to chemical reactions \[1\].

## Polyunsaturated Fats and Oxidative Rancidity

Because of their multiple double bonds, polyunsaturated fats are prone to oxidative rancidity \[1\]. This process involves the formation of free radicals from the unsaturated fat molecules, which then react with oxygen — forming a chain reaction that ends with the production of foul-smelling compounds such as ketones and aldehydes \[1\].

You may have noticed that liquid vegetable oils often develop an unpleasant smell over time. This is because they go rancid quickly. They are not suitable for regular human consumption. Even if the oil is not visibly rancid at the time of eating, oxidative rancidity occurs easily inside the warm, oxygen-rich environment of the human body. When polyunsaturated fats are consumed, they spontaneously oxidise, spreading free radical chain reactions inside the cells. The more unsaturated the fat, the more vulnerable it is to this process \[1\].

Before the health industry began promoting polyunsaturated oils as food, they were used industrially — in paint. Polyunsaturated oils work beautifully for paint. They are far less suitable for your body.

## How Polyunsaturated Oils Harm the Body — and the Thyroid in Particular

The free radical processes triggered by oxidative rancidity harm cells in several ways: they inhibit the cells' ability to produce energy, and they contribute to cell aging and chronic inflammation \[2\].

Your thyroid gland is especially vulnerable to this damage. Polyunsaturated fats very powerfully inhibit thyroid function, contributing to weight gain, fatigue, and all the other symptoms of low thyroid function. They also promote systemic inflammation, suppress immune function, damage the digestive system, and drive hormonal imbalances including hypothyroidism and insulin resistance.

I cover all of this in detail in my book [How to Boost Metabolism with Food](/books/how-to-boost-metabolism-with-food) and my [Metabolic Food Mastery™](/courses/metabolic-food-mastery) course — including how to recognise the signs of PUFA-related thyroid suppression and what to do about it.

## What Fats Should You Eat?

Given the damage caused by polyunsaturated fatty acids, it is very important to limit their intake as much as possible. This is not straightforward, because polyunsaturated fats are present in almost all foods to varying degrees — including many foods considered "natural" and "healthy."

In my [courses](/courses), you will learn how to identify polyunsaturated oils, which foods are naturally high in them, and how to build a diet centred on genuinely thyroid-supportive fats. You will also find guidance on how to safely reduce your body's accumulated PUFA stores — a process that takes time and requires a specific approach.

If you would like personalised support implementing these dietary changes, I am available for one-to-one [online nutritional and metabolic counseling sessions via Google Meet](/counseling). These sessions are tailored to your individual situation, health history, and goals.

## Why Saturated Fats Like Butter and Coconut Oil Support Your Thyroid

The narrative that saturated fats are dangerous has been thoroughly debunked by independent researchers, yet it persists in mainstream dietary advice. The truth is that butter and coconut oil are not just harmless — they are actively supportive of thyroid function and metabolic health.

**Coconut oil** is composed predominantly of medium-chain fatty acids (MCFAs), primarily lauric acid. Unlike long-chain polyunsaturated fats, these medium-chain fats are absorbed directly into the portal circulation and delivered rapidly to the liver, where they are converted into energy rather than stored as fat. Crucially, coconut oil does not suppress thyroid function. In fact, it appears to actively support the thyroid by improving the efficiency of T4-to-T3 conversion — the key step that makes thyroid hormone biologically active in your cells.

Dr. Ray Peat, whose research underpins much of the pro-metabolic nutritional framework (which I teach in my book [How to Boost Metabolism with Food](/books/how-to-boost-metabolism-with-food)), consistently highlighted coconut oil as a thyroid-supportive fat. His work showed that the medium-chain saturated fats in coconut oil increase cellular energy production, raise basal metabolic rate, and help to counteract the metabolic suppression caused by accumulated polyunsaturated fats in the body's tissues.

**Butter** is rich in short- and medium-chain saturated fatty acids and contains a profile of fat-soluble nutrients that are essential for hormonal and metabolic health: vitamins A, D, and K2, as well as butyrate — a short-chain fat that feeds the intestinal lining, reduces inflammation, and supports the liver. Vitamin A in particular is essential for thyroid hormone receptor sensitivity. Without adequate [vitamin A](/blog/ray-peat-vitamin-a), your cells cannot respond properly to thyroid hormone even if your thyroid is producing it normally. Butter from grass-fed cows is especially rich in these nutrients.

Both butter and coconut oil are completely stable at cooking temperatures. Unlike polyunsaturated oils, they do not oxidise or form free radicals when heated — making them the only genuinely safe fats to cook with. Their stability inside the body is equally important: they do not accumulate in cell membranes in ways that impair cellular signalling or energy production.

If you are working on recovering your thyroid and metabolic function, replacing polyunsaturated oils with butter and coconut oil in your daily cooking is one of the most impactful dietary changes you can make. You can learn exactly how to implement this shift — and everything else to consider alongside it — in my book [How to Boost Metabolism with Food](/books/how-to-boost-metabolism-with-food) and my [Metabolic Food Mastery™](/courses/metabolic-food-mastery) course.

## Frequently Asked Questions

### Are polyunsaturated fats really bad for you?

We are told by health authorities and the media that eating polyunsaturated fats in place of saturated fats is healthy and protective against heart disease.

In our [Hypothyroid Treatment Program](/courses/metabolic-food-mastery), you can learn how polyunsaturated fats promote inflammation, suppress immune function, harm your digestive system, and lead to hormonal imbalances including hypothyroidism and insulin resistance.

### What oils are safe to cook with?

Polyunsaturated fats are liquid oils such as corn, canola (rapeseed), soy, safflower, sunflower, flaxseed (linseed), sesame, and peanut. Saturated fats are found in for instance coconut oil and butter.

### Does coconut oil raise cholesterol - and is that a problem?

It is true that eating saturated fats increase cholesterol, but there is no scientific evidence for connecting cholesterol to heart disease.

## References

1.  [Dynamic Science — Oxidative Rancidity in Unsaturated Fats](http://www.dynamicscience.com.au/tester/solutions1/chemistry/organic/Lesson3%20ranciditysoln.pdf)
2.  [Thyroid Research Journal — Polyunsaturated Fats and Thyroid Function](https://thyroidresearchjournal.biomedcentral.com/articles/10.1186/1756-6614-5-25)
3.  [PubMed — Coconut Oil and Medium-Chain Fatty Acids in Energy Metabolism](https://pubmed.ncbi.nlm.nih.gov/1386252/)

## Related articles

-   [How to Fight Chronic Inflammation: A Pro-Metabolic Perspective](/blog/inflammation-ray-peat)
-   [Who Is Dr. Ray Peat?](/blog/who-is-dr-ray-peat)
-   [Does Vitamin A Support Thyroid Health? Dr. Ray Peat's Research Explained](/blog/ray-peat-vitamin-a)

**Tags:** polyunsaturated fats, PUFA, thyroid, Ray Peat, cooking oils, metabolism, hormones, inflammation

Source: https://biochemnordic.com/blog/polyunsaturated-fats-thyroid-ray-peat