Thyroid function is intrinsically linked to insulin sensitivity and endothelium-dependent vasodilation in healthy euthyroid subjects.
BACKGROUND
Levels of TSH respond to fluctuations in serum free T(4) (fT(4)) but remain in a very narrow individual range. There exists current controversy regarding the upper limit of normal serum TSH values above which treatment should be indicated.


OBJECTIVE
We aimed to study whether the individually determined fT(4)-TSH relationship was associated with plasma lipids, insulin sensitivity, and endothelial dysfunction in healthy subjects with strictly normal thyroid function according to recent recommendations (0.3-3.0 mU/liter).


METHODS
This was a cross-sectional study.


METHODS
The study consisted of a cohort of healthy men from the general population (n = 221).


METHODS
Oral glucose tolerance, insulin sensitivity (S(I), minimal model), endothelium-dependent vasodilation (high-resolution ultrasound), and plasma lipids were measured in relation to thyroid function tests.


RESULTS
Both serum TSH and fT(4).TSH product were positively associated with fasting and postload insulin concentration and negatively with S(I). After body mass index stratification, these associations were especially significant among lean subjects. Serum TSH and fT(4).TSH product also correlated positively with fasting triglycerides and negatively with high-density lipoprotein cholesterol. In a multiple linear regression analysis, age (P = 0.007) and S(I) (P = 0.02) but not body mass index, fasting triglycerides, or serum high-density lipoprotein concentration contributed independently to 3.7 and 3.3%, respectively, of the variance in fT(4).TSH. Those subjects over the median of fT(4).TSH showed reduced endothelium-dependent vasodilation.


CONCLUSIONS
Thyroid function tests are intrinsically linked to variables of insulin resistance and endothelial function. It is possible that underlying factors lead simultaneously to increased serum TSH, insulin resistance, ensuing dyslipidemia, and altered endothelial function even within current normal TSH levels.
Fernández-Real
Jose-Manuel
JM
Unit of Diabetes, Endocrinology, and Nutrition, Hospital de Girona Dr. Josep Trueta, Ctra. França s/n, 17007 Girona, Spain. uden.jmfernandezreal@htrueta.scs.es
López-Bermejo
Abel
A
Castro
Antoni
A
Casamitjana
Roser
R
Ricart
Wifredo
W
D001916:Brachial Artery / Q000502:physiology; D008076:Cholesterol, HDL / Q000097:blood; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D004730:Endothelium, Vascular / Q000502:physiology*; D045462:Endothelium-Dependent Relaxing Factors / Q000494:pharmacology; D005951:Glucose Tolerance Test; D006801:Humans; D007333:Insulin Resistance / Q000502:physiology*; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D005996:Nitroglycerin / Q000494:pharmacology; D013961:Thyroid Gland / Q000502:physiology*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014280:Triglycerides / Q000097:blood; D014664:Vasodilation / Q000502:physiology
D008076:Cholesterol, HDL; D045462:Endothelium-Dependent Relaxing Factors; D014280:Triglycerides; D013972:Thyrotropin; D005996:Nitroglycerin; D013974:Thyroxine