Free thyroxine, free triiodothyronine, and thyrotropin concentrations in hypothyroid and thyroid carcinoma patients receiving thyroxine therapy

Abstract

Free thyroxine (FT4) and free triiodothyronine (FT3) concentrations in serum were measured by direct equilibrium dialysis methods in patients receiving thyroxine replacement or suppression therapy. Four of 50 hypothyroid patients euthyroid on replacement therapy (mean thyroxine dose 120 \g=m\g/day) had supra\x=req-\ normal FT4 concentrations, whereas the FT3 concentrations were normal in all. Forty-one of 56 operated thyroid carcinoma patients on suppressive therapy (mean thyroxine dose 214 \g=m\g/day) had raised FT4 concentrations, whereas the FT3 concentration was elevated in only one patient. There was a large difference in mean FT4 values for hypothyroid and thyroid carcinoma patients (17.2 vs 29.5 pmol/l), whereas the difference in mean FT3 values was small (5.0 vs 6.1 pmol/l), suggesting a decreased peripheral conversion of T4 to T3 with increasing concentrations of FT4. Serum TSH concentrations, as determined by an immunoradiometric assay, varied from < 0.02 to 11.9 mU/l in treated hypothyroid patients; 21 patients (42%) had values outside the reference limits. As a single test, serum TSH is therefore not very useful for the assessment of adequate thyroxine dosage in patients with primary hypothyroidism. In thyroid carcinoma patients, the TSH concentrations were < 0.18 mU/l; 45 patients had values < 0.02 mU/l indicating sufficient suppression of TSH secretion in the majority of cases.On the basis of these results we recommend the combination of FT3 and TSH tests for monitoring thyroxine replacement and suppression therapy. FT4 appears less useful than FT3 for this purpose even if special reference values values were adopted for each patient group.Patients on therapy with thyroxine may have raised serum T4 and free T4 (FT4) concentrations but usually do not appear thyrotoxic unless serum T3 and free T3 (FT3) concentrations are also elevated (Salmon et al. 1982; Brajkovich et al. 1983;Pearce & Himsworth 1984; Mardell et al. 1985; Fraser et al. 1986). In these earlier investi¬ gations the free thyroid hormone levels in serum were studied using free hormone index or ana¬ logue-type methods. It is well-documented that in a number of situations such methods are inaccu¬ rate indicators of the true free hormone concen¬ trations (Ekins 1982; Liewendahl et al. 1984;Wilke 1986).The objective of the present study was there¬ fore to determine the usefulness of serum FT4 and FT3 concentrations, as measured by precise and accurate direct equilibrium dialysis methods, in monitoring thyroxine therapy in patients with primary hypothyroidism and patients operated for differentiated thyroid carcinoma. We also

TSH-suppressive doses of levothyroxine are required to achieve preoperative native serum triiodothyronine levels in patients who have undergone total thyroidectomy

“…Higher FT 4 levels in individuals taking L-T 4 have been shown in numerous previous studies (4,5,6,7,8). The same data were also obtained from our study and were in agreement with these studies.…”

“…Meanwhile, patients with moderately suppressed TSH levels postoperatively had higher serum FT 4 levels and unchanged serum FT 3 levels compared with their native levels, findings that are in agreement with those of Silva & Larsen (13). Indeed, most physicians encounter patients on TSH-suppressive doses of L-T 4 therapy who have serum T 4 levels higher than the normal upper limit and normal T 3 levels (2,3,5). In general, most clinicians believe that a low-serum TSH level indicates subclinical thyrotoxicosis and is a risk factor for cardiac dysfunction or osteoporosis (17).…”

“…These findings also suggest that a supraphysiological serum T 4 level was needed to normalize the serum TSH level, possibly in order to compensate for the absence of circulating T 3 secreted by the thyroid. Therefore, several investigators have advocated monitoring the dosage of L-T 4 based on the serum levels of T 3 rather than T 4 (4,5). Such a relative deficiency of T 3 in athyreotic patients on L-T 4 may be overlooked if only serum TSH and T 4 levels are determined.…”

“…Several studies regarding the use of L-T 4 therapy to treat hypothyroidism showed that when serum T 4 levels are maintained at the upper limit of the normal ranges, serum T 3 levels are within the normal ranges (4,5,6,7,8). However, there have been few studies that compared postoperative T 3 levels in patients on L-T 4 therapy with their own preoperative endogenous levels.…”

Лечение Первичного Гипотиреоза: Нерешенные Вопросы

“…Од на из них -недо сти же ние ком пен са ции за бо ле ва ния или де ком-пен са ция по сле пе ри о да стой ко го эу ти ре о за. По дан-ным раз ных ав то ров, ча с то та де ком пен са ции ги по ти-ре о за у взрос лых со став ля ет от 32,5 до 62% [7][8][9][10][11]. По ми мо пло хо го са мо чув ст вия, сни же ния ра бо то-спо соб но с ти и ка че ст ва жиз ни дли тель но не ком пен-си ро ван ный ги по ти ре оз со пря жен с ри с ком раз ви тия ос лож не ний.…”