Acute effect of exogenous thyroxine dose on serum thyroxine and thyrotrophin levels in treated hypothyroid patients

Increased fT4 concentrations in patients using levothyroxine without complete suppression of TSH

Introduction

In our hospital, physicians noticed high free thyroxine (fT4) concentrations without complete suppression of thyroid-stimulating hormone (TSH) in blood samples of patients at the outpatient clinic, which appeared to occur more often following the introduction of a new fT4 immunoassay. This discordance may be explained by incorrect reference intervals, analytical issues, or patient-related factors. We aimed to establish the contribution of the possible factors involved.

Methods

Reference intervals of both fT4 immunoassays were re-evaluated using blood samples of healthy volunteers and the new immunoassay's performance was assessed using internal quality controls and external quality rounds. The frequency of discordant fT4 and TSH pairings obtained from laboratory requests were retrospectively analysed using a Delfia (n = 3174) and Cobas cohort (n = 3408). Last, a literature search assessed whether the time of blood draw and the time of levothyroxine (L-T4) ingestion may contribute to higher fT4 concentrations in L-T4 users.

Results

The original reference intervals of both fT4 immunoassays were confirmed and no evidence for analytical problems was found. The Delfia (n = 176, 5.5%) and Cobas cohorts (n = 295, 8.7%) showed comparable frequencies of discordance. Interestingly, 72-81% of the discordant results belonged to L-T4 users. Literature indicated the time of blood withdrawal of L-T4 users and, therefore, the time of L-T4 intake as possible explanations.

Conclusions

High fT4 without suppressed TSH concentrations can mainly be explained by L-T4 intake. Physicians and laboratory specialists should be aware of this phenomenon to avoid questioning the assay's performance or unnecessarily adapting the L-T4 dose in patients.

Thyroid hormone levels affected by time of blood sampling in thyroxine-treated patients

Patients receiving exogenous levothyroxine are reported to have higher total and free serum thyroxine levels than euthyroid controls. This may be an artifact of the serum collection time. We explored the effect of collection time on serum levels of thyroid hormones in outpatients receiving levothyroxine for replacement therapy (26 patients) or suppression of thyrotropin (25 patients). Blood samples, obtained during regular clinic visits (random samples) and at more than 22 h from ingestion of levothyroxine (trough samples), were assayed for total and free thyroxine, triiodothyronine, and thyrotropin. Four athyreotic patients on levothyroxine therapy had serial blood sampling over 24 h. Compared to corresponding trough samples, random samples had elevated total thyroxine levels in patients receiving replacement (8.1 +/- 1.2%, mean +/- SE, p = 0.0001) and in patients undergoing suppression (8.8 +/- 1.6%, p = 0.0001). Free thyroxine was increased by 12.7 +/- 2.6% (p = 0.0003) and 14.5 +/- 2.3% (p = 0.0001), respectively, compared with trough samples. Thyrotropin levels were 18.9 +/- 6.8% (p = 0.003) lower in patients receiving replacement and triiodothyronine levels showed small or no changes. Time-course analysis showed that free and total thyroxine levels remained significantly elevated above baseline for 9 and 5 h, respectively, after a levothyroxine dose. In conclusion, there is a transient increase in thyroid hormone levels for 9 h after an oral levothyroxine dose. Accurate assessment of thyroid hormone levels in patients receiving levothyroxine therapy should take this into account. This has greatest significance in selecting minimal levothyroxine dosages for suppression of thyrotropin.

Cardiac systolic time intervals and thyroid hormone levels during treatment of hypothyroidism

This study was undertaken to compare results of modern serum thyroid hormone assays with cardiac systolic time intervals (STI) during thyroxine treatment in hypothyroid patients. The patients were assessed clinically (Billewicz index) and the STI and serum thyrotropin (TSH), total and free thyroxine (T4) and total and free triiodothyronine (T3) were determined in 16 hypothyroid women (Group I) treated with 50 micrograms increments of thyroxine, and in 13 women who had a history of thyroid carcinoma and high-dose thyroxine replacement therapy and had elevated thyroid hormone concentrations (Group II). The STI of 24 matched healthy female controls were used for reference of STI. The pre-ejection period (PEP) index and the PEP/LVET ratio (left ventricular ejection period) were greater in untreated overtly and mildly hypothyroid patients (p less than 0.05) than in the controls. During stable thyroxine therapy [mean daily dosage for Group I 137.5 (7.3) micrograms and for Group II 220 (61) micrograms] the PEP correlated with serum free T4 (FT4), as measured by a two-step method (SpectriaR) (r = -0.55, p less than 0.01, n = 29) and total T4 (r = -0.51, p less than 0.05, n = 29), but not with TSH, T3, FT3 or FT4 measured by an analogue method Amerlex-M(R). The TRH test was not valuable in follow-up because of the strong correlation between basal TSH and stimulated TSH values (r = 0.95). In conclusion, STI are useful for assessment of the thyroid state in untreated hypothyroid patients. Serum TSH becomes normal in the same time as STI and is the best for follow-up. If serum TSH is low and the patient is on stable thyroxine therapy, we recommend serum FT4 for monitoring thyroxine replacement. Two-step FT4 assays had the best correlation with STI, which has significance in patients with non-thyroidal illness.

Screening for thyroid disease in a primary care unit with a thyroid stimulating hormone assay with a low detection limit

In a study at a primary care centre in a predominantly rural area of Sweden the records of all patients with established thyroid disease were scrutinised and 2000 consecutive adult patients screened with an immunoenzymometric thyroid stimulating hormone assay. The aims of the study were fourfold: firstly, to assess the total burden of thyroid disease in primary care centres in Sweden; secondly, to assess the efficacy of clinical diagnosis of the disease in unselected populations of patients; thirdly, to assess the efficacy of clinical evaluation of treatment with thyroxine; and, lastly, to see whether a single analysis of the serum thyroid stimulating hormone concentration by recent methods would be enough to identify an abnormality of thyroid function. Of the roughly 17,400 adults in the study community, 111 women and 10 men were being treated for thyroid disease. Screening detected 68 patients (3.5%) not receiving thyroxine who had a serum thyroid stimulating hormone concentration of 0.20 mU/l or less, all of whom were followed up clinically. Fifty of these patients were also studied biochemically during follow up. Only nine of the 68 patients had thyroid disease (three with thyrotoxicosis requiring treatment), no evidence of the disease being found in the remainder. Sixteen patients had spontaneous hypothyroidism requiring treatment, and neither these nor three patients with thyrotoxicosis had been detected at the preceding clinical examination. Of 35 patients in whom thyroid disease was suspected clinically at screening, none had laboratory evidence of thyroid dysfunction. In this series 1.3% of all women in the study community (2.6% of all 50-59 year olds) and 0.1% of the men were being treated for thyroid disease at the primary care centre, roughly 1.0% of adults subjected to screening were found to have thyroid disease requiring treatment, and most patients with a thyroid stimulating hormone concentration of 0.20 mU/l or less did not have thyroid dysfunction. It is concluded that measuring the basal serum thyroid stimulating hormone concentration by present methods is insufficient for the biochemical assessment of thyroid dysfunction in unselected populations.

Recent advances in routine thyroid function testing

Thyroid function tests are one of the most common of endocrine laboratory investigations requested by general clinicians. The tests used therefore have to be efficient at identifying thyroid disease, monitoring treatment, and handling large numbers of tests. Recent advances in methodology have expanded both the range of in vitro thyroid function tests available and the techniques by which the well-established tests may be performed. This article reviews the methods and analytical and clinical performance of the routine tests currently available, concentrating particularly on the relatively new ones, and speculating on their role in strategies for the laboratory investigation of thyroid function.

Relevance of increased serum thyroxine concentrations associated with normal serum triiodothyronine values in hypothyroid patients receiving thyroxine: a case for "tissue thyrotoxicosis"

Fifteen patients receiving standard thyroxine replacement therapy (100-200 micrograms daily) for primary hypothyroidism and who had persistently raised free thyroxine concentrations in their serum were investigated to see whether the dose being given was too high. In addition to the usual thyroid hormone assays systolic time intervals (which indicate left ventricular contractility) were calculated as accurate reflectors of tissue thyroid activity. All patients showed the expected increased free and total thyroxine concentrations; but mean total and free concentrations of triiodothyronine were normal, while reverse triiodothyronine values were raised. Mean systolic time intervals were significantly reduced as compared with normal and fell within the thyrotoxic range. Seven patients subsequently had their doses of thyroxine reduced by 50 micrograms daily and were reinvestigated one month later. All showed significant falls in circulating thyroxine and triiodothyronine concentrations and an increase in mean systolic time intervals to the normal range. In patients receiving thyroxine replacement therapy for primary hypothyroidism a raised serum thyroxine concentration may indicate tissue thyrotoxicosis and should prompt a reduction of the thyroxine dose.