Serum and red cell folate and serum vitamin B12 levels in hyperthyroidism

Thyroid Hormone and Vascular Remodeling

Both hyperthyroidism and hypothyroidism affect the cardiovascular system. Hypothyroidism is known to be associated with enhanced atherosclerosis and ischemic heart diseases. The accelerated atherosclerosis in the hypothyroid state has been traditionally ascribed to atherogenic lipid profile, diastolic hypertension, and impaired endothelial function. However, recent studies indicate that thyroid hormone has direct anti-atherosclerotic effects, such as production of nitric oxide and suppression of smooth muscle cell proliferation. These data suggest that thyroid hormone inhibits atherogenesis through direct effects on the vasculature as well as modification of risk factors for atherosclerosis. This review summarizes the basic and clinical studies on the role of thyroid hormone in vascular remodeling. The possible application of thyroid hormone mimetics to the therapy of hypercholesterolemia and atherosclerosis is also discussed.

Prevalence of folic Acid and vitamin B12 deficiencies in patients with thyroid disorders

Although macrocytosis might occur with frequency in patients with thyroid disorders, there is controversial information on the metabolic relationship between thyroid stimulating hormone (TSH), folic acid and B12 in the general population. We performed a retrospective analysis to retrieve results of serum folic acid, B12, and TSH performed on consecutive outpatients referred by general practitioners for routine blood testing over the last 2 years. A positive, significant trend towards increased values of folic acid, but not of B12, could be observed across the spectrum of TSH values suggestive for hypo- and hyperthyroidism. However, the prevalence of subjects with folic acid or B12 deficiency did not differ significantly among the subgroups of subjects. In multivariable linear regression analysis folic acid, but not B12, was associated with TSH levels. These results do not support the routine screening for either B12 or folic acid deficiency in subjects with subclinical disturbances of thyroid function, though we can not rule out that it might still be useful in patients with overt thyroid dysfunction.

Plasma homocysteine levels in hyperthyroid patients

Hyperhomocysteinemia is a risk factor for premature atherosclerotic vascular diseases. It is known that plasma homocysteine levels are higher in hypothyroid patients compared to healthy subjects. The aim of our study was to assess plasma total homocysteine concentrations in hyperthyroid patients before and after treatment when euthyroid status was reached and compare them with control group. Thirteen hyperthyroid patients (age, 42.9 +/- 15.6 year) and eleven healthy subjects (age, 39.9 +/- 12.5 year) were involved in the study. Plasma levels of homocysteine and serum cholesterol, triglyceride, HDL cholesterol, urea, creatinine, vitamin B12, folate were measured before and after treatment. LDL cholesterol and creatinine clearances were calculated. Pretreatment homocycteine levels of the hyperthyroid patients were significantly lower than healthy controls (11.5 +/- 3.6 micromol/L vs. 15.1 +/- 4.5 micromol/L, respectively, p<0.05). Posttreatment homocysteine levels were significantly higher than pretreatment levels (13.9 +/- 6.3 micromol/L vs. 11.5 +/- 3.6 micromol/L, respectively, p<0.05) and posttreatment creatinine clearance were lower than pretreatment level (103.5 +/- 12.7 ml/min vs. 114.2 +/- 9.3 ml/min, respectively, p<0.01). Lower homocysteine levels in hyperthyroidism can be partially explained with the changes in creatinine clearance.

Homocysteine and its relation to B-vitamins in Graves' disease before and after treatment: effect modification by smoking

OBJECTIVES

To investigate plasma total homocysteine levels and its relation to B-vitamins and smoking in Graves' disease before and after antithyroid therapy.

DESIGN

A longitudinal study taking place at four hospitals in Norway.

METHODS AND SUBJECTS

Plasma total homocysteine, serum folate, serum cobalamin and riboflavin, flavin mononucleotide and flavin adenine dinucleotide in plasma were investigated in 182 patients with hyperthyroidism before treatment. The same parameters were reinvestigated in 112 of these patients after attaining euthyroid state.

RESULTS

In hyperthyroidism, plasma total homocysteine was low, and inversely related to folate, cobalamin and riboflavin, and positively related to serum creatinine and age. Following antithyroid therapy, total homocysteine increased and the concentration of folate, cobalamin, riboflavin, flavin mononucleotide and flavin adenine dinucleotide decreased significantly. The most pronounced reduction (35%) was observed for flavin mononucleotide. In the hyperthyroid state, smokers had lower levels of folate and flavin mononucleotide than non-smokers. After restoration of euthyroidism, both folate and riboflavin were significantly lower in smokers than non-smokers. Plasma total homocysteine increased according to decreasing quartiles of B-vitamins. For riboflavin, this relation was confined to smokers.

CONCLUSION

Plasma total homocysteine changes according to thyroid status. These changes may be partly attributable to altered folate, cobalamin but also riboflavin status, particularly in smokers.

New perspectives on folate catabolism

Folate catabolism has been assumed to result from the nonenzymatic oxidative degradation of labile folate cofactors. Increased rates of folate catabolism and simultaneous folate deficiency occur in several physiological states, including pregnancy, cancer, and when anticonvulsant drugs are used. These studies have introduced the possibility that folate catabolism may be a regulated cellular process that influences intracellular folate concentrations. Recent studies have demonstrated that the iron storage protein ferritin can catabolize folate in vitro and in vivo, and increased heavy-chain ferritin synthesis decreases intracellular folate concentrations independent of exogenous folate levels in cell culture models. Ferritin levels are elevated in most physiological states associated with increased folate catabolism. Therefore, folate catabolism is emerging as an important component in the regulation of intracellular folate concentrations and whole-body folate status.

Plasma total homocysteine levels during short-term iatrogenic hypothyroidism

Hypothyroidism is associated with increased cardiovascular morbidity, which cannot be fully explained by the atherogenic lipid profile observed in these patients. We have previously found elevated levels of the cardiovascular risk factor, plasma total homocysteine (tHcy), in hypothyroidism. We conducted a longitudinal study on 17 patients who had undergone total thyroidectomy for thyroid cancer. During 6 weeks of discontinued T4 substitution before radioscintigraphy (phase I), they attained a hypothyroid state, which was reversed by resupplementation (phase II). Plasma tHcy, serum creatinine, serum and red blood cell folate, serum cobalamin, and serum cholesterol were determined at 2-week intervals throughout phases I and II. There was a progressive and parallel increase in tHcy (mean, 27%), serum creatinine (37%), and serum cholesterol (100%) during phase I, and these values returned to the original level within 4-6 weeks after reinitiating T4 therapy. Serum and red blood cell folate levels showed only minor, but statistically significant, changes. In a bivariate model, serum creatinine and serum cholesterol were strongly associated with the changes observed in tHcy during short term hypothyroidism. In conclusion, we found a transient increase in both plasma tHcy and serum cholesterol during short term iatrogenic hypothyroidism, and the tHcy response is probably mainly explained by concurrent changes in renal function. The increase in both plasma tHcy and serum cholesterol may confer increased cardiovascular risk in hypothyroid patients.

Plasma total homocysteine levels in hyperthyroid and hypothyroid patients

We found a higher plasma concentration of total homocysteine (tHcy), an independent risk factor for cardiovascular disease, in patients with hypothyroidism (mean, 16.3 micromol/L; 95% confidence interval [CI], 14.7 to 17.9 micromol/L) than in healthy controls (mean, 10.5 micromol/L; 95% CI, 10.1 to 10.9 micromol/L). The tHcy level of hyperthyroid patients did not differ significantly from that of the controls. Serum creatinine was higher in hypothyroid patients and lower in hyperthyroid patients than in controls, whereas serum folate was higher in hyperthyroid patients compared with the two other groups. In multivariate analysis, these differences did not explain the higher tHcy concentration in hypothyroidism. We confirmed the observation of elevated serum cholesterol in hypothyroidism, which together with the hyperhomocysteinemia may contribute to an accelerated atherogenesis in these patients.