Abstract
Purpose: Lipid metabolism has been poorly explored in subclinical hyperthyroidism. The aim was to examine the effects of exogenous subclinical hyperthyroidism in women under levothyroxine treatment upon plasma lipids and aspects of HDL metabolism.Methodology: Ten women were studied in euthyroidism and again in exogenous subclinical hyperthyroidism. Thyroid function tests and plasma lipids were studied.Results: HDL-cholesterol (increased 21.6%, p = 0.0004), unesterified cholesterol (increased 12.3%, p = 0.04) and Lp(a) (increased 33,3%, P = 0.02) plasma concentrations were higher in subclinical hyperthyroidism compared to euthyroidism, but total cholesterol, LDL, non-HDL cholesterol, triglycerides, apo A-I, apo B were unchanged. PON1 activity (decreased 75%, p = 0.0006) was lower in subclinical hyperthyroidism. There were no changes in HDL particle size, CETP and LCAT concentrations. The in vitro assay that estimates the lipid transfers to HDL showed that esterified cholesterol (increased 7.1%, p = 0.03), unesterified cholesterol (increased 7.8%, p = 0.02) and triglycerides (increased 6.8%, p = 0.006) transfers were higher in subclinical hyperthyroidism. There were no changes in phospholipid transfers to HDL in subclinical hyperthyroidism.Conclusions: Several alterations in the plasma lipid metabolism were observed in the subclinical hyperthyroidism state that highlight the importance of this aspect in the follow-up of those patients. The increase in HDL-C and in the transfer of unesterified and esterified cholesterol to HDL, an important anti-atherogenic HDL function are consistently protective for cardiovascular health. The increase in Lp(a) and the decrease in PON-1 activity that are important risk factors were documented here in subclinical hyperthyroidism and these results should be confirmed in larger studies due to great data variation but should not be neglected in the follow-up of those patients.
Highlights
Patients with differentiated thyroid cancer submitted to total thyroidectomy are thereafter maintained for prolonged time in exogenous subclinical hyperthyroidism state by administration of levothyroxine (LT4) at supraphysiological dose regimen
We showed that the transfers of phospholipids and triglycerides to HDL were diminished in patients in subclinical hypothyroidism [9], whereas in overt hypothyroidism the transfers of all those four lipids were diminished [10]
Since patients submitted to total thyroidectomy for thyroid cancer are often maintained for long periods in subclinical hyperthyroidism, with largely ignored alterations in plasma lipid metabolism, this study was aimed to investigate these metabolic aspects that can pertain to endogenous subclinical hyperthyroidism
Summary
Patients with differentiated thyroid cancer submitted to total thyroidectomy are thereafter maintained for prolonged time in exogenous subclinical hyperthyroidism state by administration of levothyroxine (LT4) at supraphysiological dose regimen. This standard protocol is aimed to prevent tumor relapse and in many of those patients the subclinical hyperthyroidism condition is sustained for life. The plasma lipid metabolism is strongly influenced by the action of the thyroid hormones [3] so that it is noticeable that the status of this metabolism in patients with induced subclinical hyperthyroidism has been poorly explored [4]. Lipoprotein (a), [Lp(a)], that has been consistently recognized as risk factor has remained little explored in subclinical hyperthyroidism
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