Introduction:
The thyroid is part of the endocrine system, which comprises glands that produce, store, and release hormones into the bloodstream to reach the body's cells. The thyroid is an endocrine gland found at the bottom of the neck that is an H-shaped gland with various functions. The thyroid gland utilizes iodine from the meals one eats to make two main hormones: thyroid hormones (triiodothyronine (T3) and thyroxine (T4). The anterior pituitary gland secretes the stimulating thyroid hormone (TSH) that acts upon the thyroid gland to release thyroid hormones (triiodothyronine (T3) and thyroxine (T4)) that are vital for the optimal development and function of the central nervous system. These hormones influence body weight regulation, energy levels, internal temperature, skin, hair, nail growth, and metabolism and are crucial to the endocrine system. In addition, thyroid dysfunction is related to many neuropsychiatric conditions, especially subclinical hypothyroidism is associated mainly with cognitive impairment.
What Is Dementia?
Dementia is a collection of clinical syndromes where deterioration of cognitive function, i.e., declines the person's capability to live independently and the quality of life. Dementia is a chronic or progressive condition with physical, psychological, social, and economic consequences, not only for individuals living with dementia but also for their careers, families, and society. In simple words, dementia defines the ability to process thought, which is impaired beyond the expectations of others. It is considered a consequence of the biological aging process. It results from various diseases and injuries that primarily or secondarily involve the brain, such as stroke or Alzheimer's disease (cognitive impairment disorder primarily in the older age group) associated with thyroid status, which emerged as maintenance of normal neural functions. Dementia affects recollective power, thinking, orientation, comprehension, calculation, learning capacity, language, reasoning, behaviors, communication abilities, and judgment, but the consciousness remains unaffected. It is preceded by mood alteration, emotional control, behavior, or motivation. Dementia is commonly seen in older age groups and is presently the seventh leading reason of demise among all diseases. Vascular dementia (VD), also called vascular cognitive impairment (VCI), results from brain injury due to inadequate blood supply and is the second most common form of dementia only after Alzheimer’s disease (AD).
What Is Hypothyroidism?
Hypothyroidism is a low level of circulating thyroid hormone levels in the serum. It may be a congenital or an acquired disease. In acquired conditions, the cause is either primary disease or hypothalamic-pituitary-axis abnormalities; Hashimoto's Disease is the most common cause of adult who have hypothyroidism. In congenital disease, cretinism is the cause in children, where a deficiency of normal thyroid secretion is characterized by physical deformity, dwarfism, and mental retardation, and often goiter. There are different causes of hypothyroidism, classified as:
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Primary: Due to failure of the thyroid gland.
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Secondary: Due to hypopituitarism(deficiency or low level of pituitary hormones).
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Tertiary: Due to failure of the hypothalamus (part of the brain which is the control coordinate center of the brain)
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Quaternary: Due to tissue insensitivity to the action of thyroid hormones.
What Are the Clinical Symptoms of Hypothyroidism?
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Cool and pale skin.
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Hyperkeratosis (Dry roughness of skin).
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Decreased sweating.
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Generalized nonpitting edema (myxedema).
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Periorbital edema (fluid retention around the eye or puffiness of the eye).
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Grave’s ophthalmopathy: degree of stares, eye protrusion, and extraocular (eye muscles) muscle weakness.
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Bradycardia (low heart rate).
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Hypertension.
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Fatigue, shortness of breath on exertion.
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Hypoventilation.
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Obstructive sleep apnea(breathing problem during sleeping).
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Constipation.
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Weight gain.
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Ascites (fluid retention in the abdomen).
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Sleepiness.
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Carpal Tunnel Syndrome is paresthesia's of the hands and feet (often due to deposit of proteinaceous ground substance in the ligaments near the wrist and ankle)
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Forgetfulness.
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The slowing down of the relaxation phase of deep tendon reflexes.
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Hypothermia(low temperature of the body).
How Does Thyroid Function Impacts Dementia Risk?
The thyroid hormones regulate the body's metabolism. Thyroid function can be affected by various dietary micronutrients, like iodine and vitamin B12.
Reduced thyroid hormones and their function, called hypothyroidism, delay the metabolism, leading to weight gain, while elevated thyroid activity, called hyperthyroidism, increases the metabolism, leading to weight loss. At the same time, the effects on body weight are often the most visible changes to thyroid function.
The demanding organ- the brain, is susceptible to thyroid hormone levels. Low thyroid hormone during development can result in mental retardation. Clinical hypothyroidism in adults is a reason for cognitive impairment, which can be reversible upon restoring thyroid hormone levels. In subclinical hypothyroidism, there are signs of reduced thyroid activity, but thyroid hormone levels are beyond the threshold for a clinical diagnosis.
Thyroid hormones are important for normal brain evolution:
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Neurogenesis (neuron formation).
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Neuronal and glial cell differentiation and migration,
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Synaptogenesis (synapse formation).
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Myelination (outer sheath formation).
Lack of thyroid hormone during fetal and postnatal development may have a serious negative impact on the brain, resulting in delayed maturation, intellectual deficiencies, and neurological damage. Neural cells represent the thyroid hormone nuclear receptors THRA (thyroid hormone receptor alpha) and THEN (thyroid hormone receptor beta), which are mediated by triiodothyronine (T3), the active hormone. Brain T3 derives partly from the circulation and partly from type-2 deiodinase-mediated 5’-deiodination of T4 in glial cells. Membrane transporters promote the advancement of T4 and T3 across blood-brain barriers. The primary transporters are the monocarboxylate transporter 8 (MCT8), which facilitates T4 and T3 transport, and the organic anion polypeptide 1C1 (OATP1C1) transports thyroxine (T4) but not triiodothyronine (T3), it regulates the expression of many genes in the brain, mostly during developmental stages.
What Is the Treatment of Hypothyroidism Dementia?
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Levothyroxine: It is a replacement therapy to achieve an optimal level of circulating thyroid levels. Although the synthetic preparations of T4 (levothyroxine) and T3 (liothyronine). The preferred medication is levothyroxine, the recommended maintenance dose ranges from 75 to 150 mcg taken orally once daily, depending on age, body mass index, and absorption.
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Liothyronine (L-triiodothyronine): It should not be used solely for prolonged-term replacement because of its short half-life and the prominent peaks in serum T3 levels it produces, which may increase cardiac risks. Thus the cause of dementia is the hypothyroidism state. Therefore it is better to consult the medical healthcare professional to rule out the cause and recommend the specific medication. In addition, the physician will recommend some blood tests to rule out serum thyroid hormone levels for better calculating the drug dose.
Conclusion
Hypothyroidism is a metabolic disorder with a neurological aspect of cognitive dysfunction. Dementia is a general term for the degraded ability to remember, think, or make decisions that interrupt with doing everyday activities. The association between hypothyroidism state and neurological dysfunction can be reversible if diagnosed as earlier as possible with proper intervention. Thyroid disease is often a life-long medical condition that needs to be supervised by healthcare professionals with frequent follow-ups and requires daily medication.
