Informational Posts

Why Can Hypothyroidism Cause Weight Gain?

Originally published on 9th April 2016
Last updated on 1st March 2024

Thyroid disease is often used as a joke or a scapegoat for weight gain. People throw it around, and as such, it’s often not taken very seriously.

Many people think thyroid issues are just an excuse for being overweight. 

But weight gain is a legitimate symptom of an underactive thyroid or hypothyroidism, along with many others. Why? 

Rachel Sat in Front of The Sea

Why Thyroid Hormones Are Important 

Your thyroid produces the hormones T3 and T4.

Worth noting: there is also T1, T2 and Calcitonin, however, T1 and T2 are technically made outside of the thyroid gland.

These five hormones are needed for many processes, cells and functions in your body, especially T3 and T4. So when they are abnormal, i.e. low as is the case with hypothyroidism, a lot of other stuff can go wrong too.

T3 is the most active thyroid hormone, and T4 is the stored hormone.

The main purpose of thyroid hormones is to ensure the metabolism is running properly.

The metabolism’s job is to produce heat and fuel. Heat to keep us warm and fuel to give us energy. However, if we don’t have enough thyroid hormone, our metabolism won’t work properly and cannot provide us with adequate heat and fuel.

Therefore, people with an underactive thyroid or hypothyroidism can have a slow metabolism, with symptoms such as cold intolerance (from the lack of heat made), extreme tiredness, and weight gain (from the lack of calories burned to make energy).

Weight Gain in Thyroid Disease

Medical professionals seem to think that the weight gain from hypothyroidism is usually between 10 and 30 pounds, thinking that the body adjusts for the slower metabolism after while. This is very much disputed, though, as many patients carry on gaining, despite a low calorie diet.

Related Post: What Causes Unintentional Weight Loss with Hypothyroidism and Hashimotos?

So Shouldn’t the Weight Gain Stop or Come off When You Start Medication?

You would think so, yes, but it’s not always so straight forward.

It all depends on if the hypothyroidism is being optimally treated and corrected. Many thyroid patients are not on the right dose, but simply ‘in range’ as opposed to ‘optimal‘.

Remember the thyroid hormones mentioned at the start of this article? Well, we need to replace them, since the underactive thyroid isn’t making enough of them. This is done with thyroid medication, or, more accurately: thyroid hormone replacement.

Armour Thyroid Medication

Thyroid Medication Comes in a Few Forms

  • There is synthetic T4-only medication, which only contains T4
  • There is synthetic T3-only medication, which only contains T3
  • There is Natural Desiccated Thyroid (NDT) or Compounded Thyroid, which contains all five; T1, T2, T3, T4 and Calcitonin and is made from actual porcine thyroid gland

Doctors most often prefer using T4-only medication for hypothyroidism and less commonly prescribe the other options, even though they are available on prescription and can really prove useful to thyroid patients who don’t see results on T4-only meds. Why? Well, that’s up for debate. You can read a bit more about why here.

In theory, T4 should convert in to T3, but a lot of people do not convert enough T4 in to T3, so they find that their thyroid medication doesn’t work very well for them. One of the ways in which it may not work well is in relieving thyroid symptoms and correcting the metabolism, i.e weight gain.

For many people, T4 synthetics such as Levothyroxine work very well, but it’s important that we understand there is no one-size-fits-all treatment.

Optimal Levels 

Having optimal thyroid levels and not just ones that are ‘in range’ can also make a big difference to symptoms and weight. However, many people are only having their TSH and perhaps Free T4 tested, which do not show if their Free T3 is optimised, and if they’re on T4-only meds, if the T4 is converting to adequate amounts of T3. This too can leave ongoing complaints, symptoms and weight changes.

For patients kept on a thyroid medication which is not working for them, or too low a dose, this means they may be non-optimally treated, and so their symptoms may get no better. Because of this, their metabolism may not function optimally, leading to weight gain and an inability to lose weight. Along with many other hypothyroid symptoms.

However, it is also worth knowing that some people also gain weight when overmedicated for hypothyroidism. Taking too high a dose may lead to the down regulation of thyroid hormone conversion, in order to protect cells from too much T3. I have heard from thyroid patients who have their thyroid levels checked due to suspecting lowered thyroid hormone levels because of weight gain, only to find that they are being overmedicated as it is. So never guess, always test!

Adrenal Glands 

Many of us with thyroid issues may also have adrenal dysfunction.

The adrenal glands are part of the endocrine system, just like the thyroid gland. They handle many hormones that are important for a lot of bodily processes, such as handling stress. This is where cortisol is produced. Adrenal dysfunction can include elevated, lowered or mixed levels of cortisol, instead of the typical ‘cortisol slope’ we see in most people.

Our bodies can respond very interestingly to stress by causing an alteration in the cortisol slope, and this is what we refer to as adrenal dysfunction.

Having adrenal dysfunction is said to impact weight and weight management, especially weight gain around the abdomen, yet many of us with hypothyroidism or Hashimoto’s are unaware that we have less than great cortisol levels.

You can order or ask for a 24-hour saliva test, testing cortisol levels at four key points of the day, to find out if you have adrenal dysfunction. If your doctor won’t order this, you can very simply order it yourself and complete it at home. See here and here. Most doctors will only test it with a one time urine or blood sample, which is not as accurate.

Four samples taken over a 24-hour day show how your rhythm of cortisol production is working, so is the most accurate. It should be highest in the morning, tailing off throughout the day.

A History of Dieting

How many of us have embarked on restrictive diets in the past?

However, are we all aware of how this can affect our longterm ability to manage weight?

The issue of being unable to lose weight and keep it off becomes more common when someone repeatedly goes on low-calorie diets or struggles with yo-yo dieting (repeated loss and gain of weight).

What many thyroid patients don’t know is that chronic dieting can reduce Free T3 levels, the active thyroid hormone, causing the metabolism to slow down even further and weight loss to become even more difficult as time goes on.

There is also something called your weight ‘set point’ which you should take in to consideration. Your ‘set point’ is the brain’s target weight for you. It is therefore individual.

Just as the body aims to maintain a normal body temperature, it also works to maintain a body weight that is physiologically comfortable. The ‘set point’ is maintained by the hypothalamus and is often genetically influenced, however, a number of things can cause this ‘set point’ to change, including moving it to a higher or lower number.

Chronic dieting is the main cause for an abnormal ‘set point’, whereby it can result in slower metabolism. After years of dieting, you may find that your body has adjusted to needing less and less calories to maintain a specific weight.

Ways to address this can include reaching optimal thyroid levels, a more consistent diet and calorie intake (no more yo-yoing, fad diets or calorie restrictions), consistent exercise (no more overexercising followed by needing to heavily rest due to the damage going too far does) and managing your Hashimoto’s if applicable.

Related: Feeling Hungry All the Time Since Starting Thyroid Medication


To conclude, having hypothyroidism can lead to weight gain in various ways, and it is not always an easy symptom to ‘correct’.

I personally do not focus on weight loss specifically as a thyroid patient advocate and you can read why here.

Are you a thyroid patient experiencing weight fluctuations?

You can click on the hyperlinks in the above post to learn more and see references to information given.

Be Your Own Thyroid Advocate Book CoverSee also:

The book Be Your Own Thyroid Advocate: When You’re Sick and Tired of Being Sick and Tiredwhich builds on this article in detail. Reclaim your thyroid healthy life and find out what you can do to help yourself get better.



About Author

Rachel Hill is the highly ranked and multi-award winning thyroid patient advocate, writer, speaker and author behind The Invisible Hypothyroidism. Her thyroid advocacy work includes writing articles, authoring books, producing her Thyroid Family email newsletters and speaking on podcasts and at events about the many aspects thyroid disease affects and how to overcome these. She is well-recognised as a crucial and influential contributor to the thyroid community and has a large social media presence. Her bestselling books include "Be Your Own Thyroid Advocate" and "You, Me and Hypothyroidism".


  • NeelamEdge
    October 13, 2019 at 3:59 pm

    Valuable information.

  • Lucie Colwell
    October 11, 2018 at 11:25 am

    I have a naturally really large prominent thyroid. Many doctors have checked it, but they never actually show me the full results! I suspect that it’s showing low levels on some of T’s….but not the ones they commonly treat in the UK. I have a Fibromyalgia diagnosis, and am now suffering from unbearable fatigue. I’ve put on around 3 stone in weight. I can (and before our wedding 2 years ago, did…) lose weight following Slimming World, but it’s only a very little at a time (like half a pound) and I have to be ridiculously strict. One treat, off-plan meal….I’ll gain weight.
    I’m a bit at the end of my tether at the moment, after a sleep study shows no…I don’t have sleep apnea to explain the exhaustion. I really think it could be my thyroid.
    What test should I be asking for?
    Many thanks. Xx

    • Rachel Hill
      October 11, 2018 at 12:20 pm

      So sorry to read about your struggles Lucie. 😔 I would start by getting a print out of any tests so you can begin with the knowledge of what you’ve already had tested. The full thyroid panel consists of TSH, Free T3 and T4, Thyroid Antibodies TpoAB and Tgab and Reverse T3 (though extremely uncommon to successfully get done). I would be looking at your free t3 mainly to begin with and checking it is optimal (not just in range). If you haven’t already, feel free to join my Facebook group Thyroid Family. Over 36,000 members willing to help and support you further.

  • CalebSmall
    January 9, 2018 at 3:33 am

    Another great post.

    • Rachel Hill
      February 21, 2018 at 9:32 pm

      Thanks 🙂


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