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PubMed1983

Acute changes in thyroid function tests following ingestion of thyroxine.

The timing of blood sample collection in relation to ingestion of thyroxine has been thought to be of no consequence in the assessment of patients receiving thyroxine medication. We have investigated changes in serum thyroid hormones after oral ingestion of thyroxine. Therapeutic doses of thyroxine (100-300 micrograms) were given to five normal, euthyroid subjects and eleven patients receiving long-term thyroxine-replacement medication. Blood samples were collected prior to and following ingestion of thyroxine. A significant increase in total thyroxine (T4), free thyroxine index (FTI) and free T4 (FT4) concentration was observed at all doses. No significant change was observed in 3,5,3'-triiodothyronine, 3,3',5'-triiodothyronine or thyrotrophin concentrations. Although T4, FTI and FT4 were significantly elevated 1 h after ingestion of all doses of thyroxine and remained elevated for at least 6 h, supranormal values were observed only after ingestion of the highest dose of thyroxine. The levels of T4, FTI and FT4 in patients receiving thyroxine should be interpreted in relation to the time of thyroxine administration. Standardisation of blood collection in patients receiving thyroxine replacement would be desirable.

Humans↗
PubMedDate not supplied

Thyroid function tests and their interpretation.

For general screening of children who are in fairly good health and in whom CNS or pituitary disease is not strongly suspected, the newer THS assays are very useful in assessing the action of both endogenous and exogenous thyroid hormone. Those children in whom primary hypothyroidism is strongly suspected or who are already on thyroid hormone supplementation, the free T4 assay provides a useful adjunct to the TSH. If Grave's disease or factious hyperthyroidism is suspected, the total T3 assay is a useful adjunct to the TSH and the free T4. The screening of possible hypothyroidism as a result of hypothalamic or pituitary disease, the free T4 is relied on heavily, along with the response of TSH to TRH stimulation. Measurement of the nocturnal TSH surge may also be useful in this situation.

Child↗
PubMed1977

Thyroid function tests in the elderly in the community.

A study of the well elderly living at home has demonstrated that the ranges for serum thyroxine (T4), triiodothyronine (T3) uptake and free-thyroxine index (FTI) are much narrower than those for the sick elderly in-patient. The prevalence of thyroid disease appears similar in both types of elderly population.

Aged↗
PubMed1976

Effect of fatty acids on thyroid function tests in vitro and in vivo.

Addition of long-chain fatty acids to serum increased thyroxine (T4), measured by a competitive protein binding assay, and triiodothyronine (T3) uptake by Sephadex or resin (T3U tests). This is compatible with the assumption that fatty acids compete with thyroxine for binding sites on T4-binding proteins. When equimolar concentrations of various saturated and unsaturated fatty acids were added to serum it was observed that the effectiveness in raising tests based on protein binding of thyroid hormones incrreased serum T3 determined by radioimmunoassay (RIA). T4(RIA) was not significantly influenced by either saturated or unsaturated fatty acids. Serum T4(CPB) rose during storage at 22degreesC and 37degreesC but was stable at 4degreesC and --20degreesC for periods up to two weeks. The proportional increase in T4(CPB) and free fatty acids (FFA) indicated that this phenomenon was due, at least partly, to the interference from FFA formed during storage of the serum. There was also a small, significant increase in T3U, T3(RIA) and CT4I (a free thyroxine estimate) after storage of serum at room temperature or higher for one to two weeks. Serum T4(RIA) did not alter during two weeks of storage. In five subjects with raised serum FFA after eating a fat meal followed by a heparin injection an increase in T4(CPB), T3U, T3(RIA) and CT4I that was proportional to the increase in FFA was observed. This effect on the thyroid tests was small until the increase in FFA concentration exceeded 2 mmol/l. T4(RIA) did not respond to the increase in FFA. In ten patients with raised levels of FFA due to uncontrolled diabetes T4(CPB), T4(RIA) and T3(RIA) decreased while T3U increased. These unexpected alterations were probably related to the severe, chronic illness in these patients. Increased FFA in vivo seem to be of little importance for the interpretation of thyroid tests in clinical practice.

Adult↗
PubMedDate not supplied

The cost-effectiveness of three thyroid function testing strategies for suspicion of hypothyroidism in a primary care-setting.

OBJECTIVE: To determine the sensitivity and specificity of thyroxine (T4) and the cost-effectiveness of three testing strategies in the diagnosis of hypothyroidism in a primary care setting. DESIGN: 1) A retrospective chart review to determine sensitivity and specificity of T4 in diagnosing hypothyroidism; a cost-effectiveness analysis comparing ordering an initial T4 test alone, an initial thyroid-stimulating hormone (TSH) test alone, and T4 and TSH tests together in diagnosing hypothyroidism; a sensitivity analysis was performed on critical assumptions. SETTING: Primary care adult practice of a health maintenance organization. PATIENTS: Eight hundred sixteen consecutive patients suspected of having hypothyroidism who had both T4 and TSH tests performed. INTERVENTIONS: None. RESULTS: The sensitivity of a T4 cut-off of 7 micrograms/dl (90.3 nmol/L) in diagnosing primary hypothyroidism was 93% (95% confidence interval = 85-100%) and the specificity was 68% (95% confidence interval = 65-71%). The cost-effectiveness ratios of using an initial T4 or TSH test were about the same across a wide range of test characteristics and disease prevalence estimates. As the ratio of T4 to TSH test charges declines from 0.6 to 0.2, the marginal cost of the TSH-first method increases from $3,500 to $18,000 for each additional hypothyroid patient identified. Ordering both tests together was very costly compared with the single test methods ($125,000 for each additional case diagnosed) and remained so under a wide range of assumptions. CONCLUSIONS: When hypothyroidism is suspected, a TSH-first testing approach is generally preferable due to its greater sensitivity and, under most assumptions, only small increment in average or marginal cost per case compared with a T4-first method.

Cost-Benefit Analysis↗
PubMed1988

The SimulTRAC FT4/TSH assay evaluated as a first-line thyroid-function test.

We evaluated the SimulTRAC FT4 57Co/TSH 125I dual-isotope assay for the simultaneous measurement of free thyroxin (FT4) by radioimmunoassay analog techniques and of thyrotropin (TSH) by immunoradiometry. Inter- and intra-assay CVs were less than 10% over the entire range tested except for 15.9% at the lowest FT4 concentration. Results obtained by the SimulTRAC assay allowed complete differentiation of 85 hyperthyroid patients and 35 hypothyroid patients from normal subjects. However, such estimations of FT4 or TSH concentrations occasionally were misleading for assessing thyroid status in various clinical conditions. We conclude that the SimulTRAC assay has the same inherent disadvantages possessed by FT4 analog and TSH immunoradiometric assays; however, where results of one of the simultaneous assays may be misleading, the results provided by the other may indicate the underlying pathology without requiring an additional assay.

Adult↗
PubMed1987

Changes in thyroid function tests and sex hormone binding globulin associated with treatment by gonadotropin.

Increasing levels of E2 in gonadotropin-treated women stimulated hepatic synthesis of SHBG and TBG, and consequently increases in T4 concentration. Nevertheless, unchanged FTI and TSH suggested that a euthyroid state was maintained. The temporal patterns for the rise in serum concentrations of TBG and SHBG during gonadotropin therapy suggest that the synthesis of these proteins by the liver has different sensitivities to E2.

Chorionic Gonadotropin↗
PubMed1984

Magnetic antibody immunoassay thyroid function tests in general practice.

Commercial radioimmunoassay kits for the measurement of human total thyroxine (T4) and total triiodothyronine (T3) based on magnetic separation are discussed for use in dogs and cats. The T4 assay procedure is modified to increase the accuracy over normal canine and feline ranges using four standards from kit components provided. Results of validation are presented and indicative normal ranges given: canine T4, 0.94 to 3.45 micrograms/dl; canine T3, 0.36 to 1.52 ng/ml; feline T4, 1.34 to 4.70 micrograms/dl; feline T3, 0.23 to 0.95 ng/ml. There is a significant breed difference in T4 levels between poodles and Yorkshire terriers.

Animals↗
PubMed1983

Fasting decreases thyrotropin responsiveness to thyrotropin-releasing hormone: a potential cause of misinterpretation of thyroid function tests in the critically ill.

We have previously reported that caloric deprivation inhibits peripheral T4 metabolism and blunts the TSH response to TRH in euthyroid obese subjects. To determine whether these phenomena also occur in hypothyroid subjects, T4, T3, rT3, and the TSH response to TRH were measured initially and after a 60-h fast in seven hypothyroid patients. Short term fasting caused a 29% decrement in the maximum serum TSH increment and a 32% decrement in the integrated TSH response to TRH (P less than 0.01). In two subjects with mild hypothyroidism, basal TSH as well as the TSH response to TRH were reduced to levels within the normal range. Specifically, basal TSH values decreased from 7.6 to 3.5 microU/ml and from 11 to 4.1 microU/ml. In the seven subjects, mean serum T3 decreased significantly from 88 to 60 ng/dl, (P less than 0.05) and rT3, initially undetectable in six of seven subjects, rose to detectable or low normal values in four of seven subjects, serum T4 remained at 2.7 micrograms/dl during both study periods. We conclude that 1) fasting induces changes in both peripheral thyroid hormone metabolism and the hypothalamic-pituitary axis in hypothyroid individuals which are qualitatively similar to those that occur in euthyroid subjects; and 2) in certain hypothyroid subjects, fasting alone can decrease basal TSH values to within the normal range. If these data can be extrapolated to critically ill subjects whose caloric intake may be diminished, they suggest that basal TSH concentrations in moderately and severely hypothyroid critically ill subjects will accurately reflect the biochemically hypothyroid state. However, mild degrees of hypothyroidism in critically ill subjects might be overlooked due to the lowering effect of fasting or poor caloric intake alone on basal TSH concentrations.

Adult↗
PubMed1994

Thyroid function tests--the next generation.

The Council on Scientific Affairs of the California Medical Association presents the following inventory of items of progress in internal medicine. Each item, in the judgment of a panel of knowledgeable physicians, has recently become reasonably firmly established, both as to scientific fact and important clinical significance. The items are presented in simple epitome, and an authoritative reference, both to the item itself and to the subject as a whole, is generally given for those who may be unfamiliar with a particular item. The purpose is to assist busy practitioners, students, researchers, and scholars to stay abreast of these items of progress in internal medicine that have recently achieved a substantial degree of authoritative acceptance, whether in their own field of special interest or another. The items of progress listed below were selected by the Advisory Panel to the Section on Internal Medicine of the California Medical Association, and the summaries were prepared under its direction.

Humans↗
PubMed1989

Thyroid function tests in children with congenital hypothyroidism on L-thyroxine treatment.

The plasma levels of thyroxine (T4), triiodothyronine (T3), free T4 (FT4), free T3 (FT3), reverse T3 (rT3) and immunoradiometrically assayed thyrotropin (IRMA TSH) have been measured in 28 L-T4-treated children with congenital hypothyroidism as well as in a control group (group C). The patients were subdivided into 2 groups according to the nonsuppressed (group A) or suppressed (group B) TSH response to TSH-releasing hormone (TRH). Basal IRMA TSH correlated with the TSH increment after TRH and it was significantly lower in group B vs. groups A and C, while no difference was present between groups A and B in regard to T4, FT4 and rT3, all higher than in group C. FT3 levels were similar in the 3 groups. In children, as in adults, basal IRMA TSH seems to be a reliable index in monitoring overtreatment.

Adolescent↗