Is There Any Effect on Smell and Taste Functions with Levothyroxine Treatment in Subclinical Hypothyroidism?

This is all more understandable that in the majority of respondents olfactory functions on discrete scents of mint and anise were not disturbed, though we found also in all this cases delayed latencies of cortical potentials PN1  and  PN5. However, differences were noticed in the objective tests in which evaluation concerned recordings of electric responses to olfactory stimulation of nerves N1 and N5 by means of aromatic smells, mint and anise in groups of healthy subjects and hypothyroidism. The ERA 2250 apparatus by Madsen Electronics is used to record evoked responses by means of Beckman electrodes placed to the forehead and bilaterally to the nape (or the neck). The technique of summing and averaging responses to a quantitatively identical stimulus was used. Anise oil stimulated endings of the olfactory nerve and mint oil endings of the olfactory and trigeminal nerves.

However total receptor count and density, olfactory epithelium surface area and thickness were found not to differ in relation to exposition with PTU. Twenty eight subclinical hypothyroid patients, and 31, synthroid competitors age, gender, education level matched participants enrolled in the study. The control group was selected on the basis of the biochemical evidence of a normal thyroid function (TSH and fT3/fT4). The conversion rate to overt hypothyroidism was found between 2% and 6% in subclinical hypothyroid patients. Higher TSH levels or presence of anti-thyroid peroxidase antibodies (Anti-TPO) increase the conversion risk 1.

• The technique of summing and averaging responses to a quantitatively identical stimulus was used.
• In the subjective olfactory tests performed using Ellsberg’s olfactory test method modified by Pruszewicz in the hypothyroid patients, the smell perception thresholds (mint and anise) were normal in 85% cases.
• Descriptive analyses were presented as means ± standard deviations for normally distributed variables.
• Recent work by Clark et al. 18 indicated that tasting of bitter compounds would modulate thyrocyte function and T3/T4 production.

Making use of the impulse olfactometer with the electronically amplified olfactory stimulus (designed by Giesen and Mrowiński), Alber et al. (1972) made computerized averaging of 1.5- second electroencephalogram sections in responses to an olfactory stimulus 25. Then, Herberhold obtained cortex evoked olfactory potentials using 10–20 mL stimuli with 100–200 ms stimulation time. He obtained two separate potentials of 250 ms latency time for responses from the trigeminal nerve and 500 ms for those from the olfactory nerve 8.

Defects of taste and smell in patients with hypothyroidism

• To do this, you can order a complete thyroid blood panel to see if your thyroid hormones are standard, high, or low.
• It was possible to differentiate responses to stimuli irritating nerve V endings (potential PN5 within latency range 200–410 ms) as well as nerve I endings (potential PN1 within latency range 460–700 ms).
• Differences between numeric variables of two groups were tested with independent samples Student’s t-test for continuous variables displaying normal distribution and Mann—Whitney U test for continuous variables not displaying normal distribution.
• Treatment with throid hormones largely reversed both the taste and smell defects.
• The subjective Ellsberg method modified by Pruszewicz was used primarily for selection of the control group and to obtain preliminary information on the state of the organ of smell in the other participants of the experiment.

Prevalence of subclinical hypothyroidism is relatively high among elderly and women. Our taste and smell sensations have their own receptor organs, but they are closely connected. Likely, your ability to taste your food is also reduced until your nasal congestion resolves. When food doesn’t taste right, it can alter your appetite and change your eating habits.

Table 2. Olfactory and Taste results of Patients (before and after treatment) and Controls.

The subjective Ellsberg method modified by Pruszewicz was used primarily for selection of the control group and to obtain preliminary information on the state of the organ of smell in the other participants of the experiment. Thresholds of perception for those with a normal sense of smell determined by Pruszewicz for both oils of mint and anise are 12 mL. It is the volume of the saturated vapors of these oils at room temperature about (22 ± 1)°C administered with a syringe in about 0.5 sec into each nostril separately. Identical quality and the production of these oils according to the manufacturer—The National Chemical Reagents POCh—are guaranteed for 50 years 9, 15. In the control group the scope of the thresholds of perception for both anise and mint oils ranged in 3–8 mL. Świdziński was used with similar stimulus lasting for 0.5 s with speed of 10–30 mL/s.

According to a 2018 review in the World Journal of Otorhinolaryngology Head and Neck Surgery, studies and reviews show that more than 350 drugs can cause changes in taste, and more than 70 drugs can cause changes in smell. Our study included 31 controls and 28 untreated subclinical hypothyroid patients. Mean age was 29±8.5 years in the control group and 29.9±9.0 years in the study group. Groups did not differ significantly in terms of age, sex and education level (Table 1).

The studies by Mackay-Sim and Beard conducted on mice indicate that thyroxine is necessary for normal development of the nervous system, including the genesis of new olfactory receptor neurons 4, 5. Although hypothyroidism disrupts development of the olfactory epithelium, it does not cause however complete atrophy of neurons 6. Olfactory disorders are most frequently caused by conduction disorders of the sensory stimulus mainly due to upper respiratory tract infections, infections of the nose and sinuses, and injuries or as an idiopathic disorder 7–9. The symptoms of a nervous system disorder, most frequently in the form of peripheral neuropathy, do not manifest themselves in a way that significantly impairs the patient’s ability.

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A subjective olfactory perception does not always correlate with objective tests of the central nervous system (CNS) responses 18. Bajaj 6 et al. was searched for cognitive functions in subclinical hypothyroid patients. This study revealed that increasing levels of TSH correlate with the decline in cognitive functions. Animal studies revealed global affection of brain from hypothyroidism in terms of weight loss and cognitive decline.

Correlation analysis were made with Pearson’s and Spearman’s correlation analyses, respectively. Some people also attribute changes in taste to the medication used to treat thyroid conditions, such as thyroid hormone replacement medications. For example, some reports indicate that it is not uncommon to have a metallic taste in your mouth if you take levothyroxine. However, one study found that altered taste improved significantly in the study group after being on levothyroxine for three months. The fact is we don’t know exactly why an altered sense of taste can be a symptom of a thyroid issue. But, one more recent study of hypothyroidism shows evidence that low thyroid hormone can lead to burning mouth syndrome and an altered taste sensation.

Results of the correlation between TSH and latency of smell cortex evoked potentials PN1 at mint oil stimulation. Sensitivity for “bitter” was significantly decreased in hypothyroid patients compared to controls. Recent work by Clark et al. 18 indicated that tasting of bitter compounds would modulate thyrocyte function and T3/T4 production. Thus, the present findings could be the result of a complex network between taste and thyroid functions. Tests for smell, taste and endocrine function of patients and controls were recorded before and after treatment. Subclinical hypothyroid patients exhibited a significantly decreased olfactory sensitivity; in addition, bitter taste was significantly affected.