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Have you ever gone looking for the source of a smell just to find out that you were the only one that could smell it? Phantom smells, also known as olfactory hallucinations, become more common as we age. In fact, 1 out of every 15 people over the age of 40 reports having them. While aging is a common cause, phantom smells can be a sign of underlying medical conditions or endocrine disorders such as hypothyroidism.
Hypothyroidism is a deficiency of thyroid hormones - thyroxine (T4) and triiodothyronine (T3). This deficiency is commonly due to damage to your thyroid gland which releases thyroid hormones. Damage could be due to an autoimmune condition (Hashimoto’s thyroiditis) or a medical procedure like thyroid surgery or radiation.
As your thyroid hormone levels drop, your pituitary gland releases more thyroid stimulating hormone (TSH) to tell your thyroid gland to release more thyroid hormone. But the damage to your thyroid gland prevents this from happening. Thus, those with hypothyroidism generally have high levels of TSH with low levels of thyroid hormone. Symptoms of hypothyroidism include:
- Feeling cold
- Weight gain
- Memory changes or “brain fog”
- Mental health changes
- Hair and skin changes
Here, we’ll learn about smell disorders and their link with hypothyroidism. But first, let’s review how our sense of smell works.
Our senses of smell and taste are part of our chemosensory system or chemical senses. Your sense of smell allows you to enjoy your favorite foods and warns you of possible dangers like fire. But, how does it work?
Specialized sensory cells called olfactory sensory neurons live inside your nose. When these neurons detect an odor they send a signal to your brain which identifies the smell. Smells can reach your olfactory sensory neurons through your nose or a channel that connects the top of your throat to your nose.
Taste and smell connection
As you chew your food, aromas travel to your olfactory sensory neurons through the connection channel. If the aromas can’t get through the channel, your sensory neurons won’t become activated. Thus, causing your food to taste bland or flavorless. As you can see, your sense of taste and smell work together.
Common causes of smell disorders
A smell disorder is when a person’s ability to smell decreases or their perception of smells changes. For instance, cookies baking no longer smell sweet but rather foul.
Most people develop smell disorders after a head injury or an illness, especially a sinus or upper respiratory infection because congestion can block the channel. Other causes include:
- Smoking cigarettes
- Hormone imbalances
- Dental problems
- Medications such as antibiotics or antihistamines
- Nervous system disorders (Parkinson’s disease, Alzheimer's disease)
Types of smell disorders
There are four types of smelling disorders:
- Hyposmia: a decrease in one’s ability to detect an odor
- Anosmia: a complete inability to smell
- Parasomnia: change in perception of how something smells
- Phantosmia: smelling an odor that isn’t there
It is hard to determine the exact number of people who have a smell disorder as most cases are self-reported.
Phantosmia is a type of hallucination and the most common self-reported one among healthy individuals. Phantosmia affects more women than men. Episodes last 5 to 20 minutes and will go away on their own.
Phantom smells are usually unpleasant such as:
- Cigarette smoke
- Rotting foods
- Something burning
- Dirty or musty
Very few people report smelling sweet or savory things. While most only have one phantom smell, it is possible to have more than one.
Hypothyroidism changes your smell perception and in some cases suppresses it all together. A 1975 study looked at taste and smell functions in people with primary hypothyroidism. Before starting thyroxine treatment, 7 out of the 18 people had changes in their smell. With T4 treatment, all patients saw improvements in their sense of smell.
A 2015 study showed that people with primary hypothyroidism had trouble identifying odors and telling them apart when compared to those without hypothyroidism. Free T3 levels had a stronger correlation with smelling dysfunction compared to TSH or T4.
A more recent study showed that the higher the TSH level, the longer it took for the brain to identify a smell. When TSH values were really high (above 30 μlU/mL), smells couldn’t be identified.
Why does this occur?
Researchers don’t fully understand the connection. One study suggests that phantosmias start from a disruption in the interaction between the brain and the olfactory neurons. Hypothyroidism has an effect on both your cognitive function and neurons.
Animal studies show that neuron development needs T4. In the absence of enough T4, your olfactory epithelium which contains olfactory sensory neurons doesn’t develop, thus potentially causing a smell disorder.
Even though the exact cause is unknown, there appears to be many pathways where hypothyroidism can impact your sense of smell.
The good news is that hypothyroidism is manageable with thyroid hormone replacement medications. While most saw improvements in their sense of smell with T4 treatment over time, not all smell defects will go away.
If you are already taking thyroid hormone replacement medications and develop phantom smells, reach out to your thyroid provider. They may want to measure your thyroid levels to make sure that your dose is correct.
Having unpleasant, phantom smells? Then it is time to get your thyroid hormone levels checked. Our at-home test kit is easy to use and will have your results back within a week. Our thyroid providers can review your results with you and determine next steps.