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Which thyroid‐stimulating hormone level should be sought in hypothyroid patients under l‐thyroxine replacement therapy?

We sought to determine whether relationships exist between level of thyroid‐stimulating hormone (TSH) suppression and cardiovascular risk parameters such as plasma homocysteine, C‐reactive protein (CRP), fibrinogen, d‐dimer and serum cholesterol in patients taking l‐thyroxine‐replacement therapy (LT4‐RT). Four hundred and two hypothyroid patients under LT4‐RT were cross‐sectionally evaluated. Patients were grouped according to their achieved TSH (mIU/l) levels under LT4‐RT on an arbitrary manner. Those patients having a TSH value 0.4–2 were defined as group 1 (n = 154), a TSH level of 2 to less than 5.5 were defined as group 2 (n = 176) and a TSH level of 5.5 to less than 20 with a normal free T3 and free T4 level were defined as group 3 (n = 72). The three groups of patients were also compared with overt hypothyroid patients (n = 71) and healthy controls (n = 97). Homocysteine levels (μmol/l) were significantly different between the three groups (10.4 ± 4 for group 1, 11.3 ± 3.7 for group 2 and 13.5 ± 4.7 for group 3; p < 0.01 for all groups). Significant differences in CRP (mg /l) levels were present between the three groups (2.6 ± 2.6 for group 1, 3.3 ± 2.9 for group 2 and 4.8 ± 4.1 for group 3; p < 0.01 for all groups). Univariate analysis showed that both homocysteine and CRP levels significantly correlated with free T4 and TSH level (p < 0.01 for both groups). No statistically significant differences were noticed in respect to fibrinogen and d‐dimer levels between three groups. In examining the effect of LT4‐RT on lipid parameters, a tendency towards beneficial effects without achieving statistical significance was observed. Practically speaking, a target TSH level of less than 2 might be advisable to lower CRP levels and homocysteine levels, and possibly lipid parameters.