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Serum Total cholesterol level in Hashimoto Thyroiditis
A fifty-one year old woman presented to her internist with complaints of swelling In the front of her neck. Upon history and physical examination, her physician noted that she had a weight gain in the past year of 28 pounds, had shortness of breath, and was easily fatigued. The physician ordered a complete blood count, a basic metabolic panel, thyroid stimulating hormone (TSH) assay, and a free thyroxin (FT4) assay. Upon receipt of the laboratory reports of the elevated TSH and decreased FT4, the Physician requested Antibody testing. The results of the antibody testing indicated a markedly increased presence. With these laboratory findings and given the Clinical symptoms, the physician diagnosed Hashimoto thyroiditis. The patient was placed on L-thyroxin (levo thyroxin) as a thyroid hormone replacement therapy to prevent further thyroid destruction. In the given clinical situation, which of the following levels of serum total cholesterol should be expected for this patient?
(The serum total cholesterol in normal health ranges between 150-220 mg/dl)
A. 100-150 mg/dl
B. 250-350 mg/dl
C. 150-220 mg/dl
D. Any of the above
E. None of the above.
The correct answer is B-250-350 mg/dl.
High cholesterol levels are observed in hypothyroidism.
Hashimoto’s thyroiditis is an autoimmune disease in which the thyroid gland is gradually destroyed by a variety of cell and antibody mediated immune processes. The disease is characterized by cellular and humoral responses, as demonstrated by serum antibodies to thyroid antigens and the lymphocytic infiltration of the gland. Hashimoto thyroiditis begins as a gradual enlargement of the thyroid gland followed by a usually slow development of hypothyroidism. Patients typically present with an enlarged thyroid resulting in diffuse goiter in the anterior region of the neck. The goiter is usually asymptomatic, but occasionally patients have difficulty in swallowing and a feeling of local pressure.
Increased levels of TSH (thyroid stimulating hormone) are the first detectable abnormality seen in laboratory testing for correctly diagnosing hypothyroidism. This may occur even before the patient is symptomatic. As hypothyroidism progresses, the FT4 (free thyroxin) decreases. Serum total cholesterol levels are commonly found to be elevated in conditions of hypothyroidism.
Hypercholesterolemia in hypothyroidism
The normal range for total plasma cholesterol in humans is considered to be 150-220 mg/dl preferably it should be <200 mg/dl (or <5.2) mmol/L. Cholesterol is transported in plasma in lipoproteins, and in humans the highest proportion is found in LDL.
The LDLs (containing cholesteryl esters) are taken up by cells by a process known as receptor-mediated endocytosis (figure). The LDL receptor mediates this endocytosis and is important to cholesterol metabolism.
Figure- Receptor mediated endocytosis of LDL cholesterol
These LDL receptors are down or upregulated depending upon the intracellular cholesterol concentration. Thyroid hormone has a role to play in the internalization of cholesterol. It promotes up regulation of LDL receptors, thus more receptors are mobilized to the cell surface to internalize more and more of cholesterol. The net circulating cholesterol level goes down which is why hypocholesterolemia is a finding in thyrotoxicosis and hypercholesterolemia is observed in hypothyroidism.
Thyroid hormone promotes endogenous cholesterol synthesis also by stimulating the activity of HMG Co A reductase enzyme, which is the rate limiting enzyme of cholesterol biosynthesis. But the effect is more pronounced for internalization of cholesterol. That is the reason that hypercholesterolemia is found in hypothyroidism, low thyroid, decreased LDL uptake, more circulating LDL and hence high serum cholesterol levels.
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