Thyroxine, free

MMI and PTU can lead to methimazole embryopathy with choanal or esophageal atresia. In pregnant women the antithyroid diug dose should be minimized to prevent fetal hypothyroidism by maintaining the maternal free thyroxine serum level slightly above the upper limit of normal. 

Measure a free thyroxine serum concentration at baseline and at 6- to 12-month intervals thereafter.34... 

D24. Docter, R., van Toor, H., Krenning, E. P., de Jong, M., and Hennemann G., Free thyroxine assessed with three assays in sera of patients with nonthyroidal illness and of subjects with abnormal concentrations of thyroxine-binding proteins. Clin. Chem. 39,1668-1674 (1993). 

E5. Ekins, R., Validity of analog free thyroxin immunoassays. Clin. Chem. 33,2137-2144 (1987). 

J5. Jaume, J. C., Mendel, C. M Frost, P. H., Greenspan, F. S and Laughton, C. W., Extremely low doses of heparin release lipase activity into plasma and can thereby cause artifactual elevations in serum free thyroxine concentration as measured by equilibrium dialysis. Thyroid 6, 79-84 (1996). 

Surks, M. I., Hupart, K. H., Pan, C., and Shapiro, L. E., Normal free thyroxine in critical non-thyroidal illnesses measured by ultrafiltration of undiluted serum and equilibrium dialysis. J. Clin. Endocrinol. Metab. 67, 1031-1039 (1988). 

Adverse effects on the thyroid have not been observed in children, however. In a study of inner-city children, linear regression analysis revealed that there was no association between PbB levels and either thyroxin or free thyroxin (Siegel et al. 1989). Similar findings were reported by Huseman et al. (1992) in a group of 12 children from the Omaha Lead and Poison Prevention Program with PbB levels in the range of 41 to 72 pg/dL. Siegel et al. (1989) offered four possible explanations to account for this apparent... 

P. Nuutila, P. Koskinen, K. Irjala, L. Linko, H.-L. Kaihola, J. U. Eskola, R. Erkkola, P. Seppcala, and J. Viikari, Two new two-step immunoassays for free thyroxine evaluated Solid-phase radioimmunoassay and time-resolved fluoroimmunoassay, Clin. Chem. 36, 1355-1360 (1990). 

Oral contraceptives have their most significant effect on endocrine parameters. Blood cortisol, thyroxine, protein-bound iodine, T3 uptake, and urinary free cortisol are elevated. Urinary 17,21-dihydroxy steroids, 17-ketosteroids, and estrogens are decreased. There is no effect on urinary catecholamines or VMA (Table 10) (LIO). The effect of thyroid functions tests is due to the administered hormone stimulating an increase in the production of thyroid-binding globulin which in turn binds 1-thyroxine. The lowering of free thyroxine stimulates the anterior pituitary to produce thyrotropin, which in turn stimulates the thyroid to produce more thyroxine. Since the additional thyroxine is bound to the extra protein, there is an equilibrium and the patient remains clinically euthyroid, but the protein-bound iodine and the thyroxine are elevated. 

D7. DeNayer, P. H., Malvaux, P., Ostyn, M., VandenSchrieck, H. G., Beckers, C., and deVissoher, M., Serum free thyroxine and binding proteins after exercise. J. Clin. Endocrinol. Metab. 28, 714-715 (1968). 

Hypothyroidism Quetiapine demonstrated a dose-related decrease in total and free thyroxine (T4) of approximately 20% at the higher end of the therapeutic dose range... 

Lab test abnormalities Phenytoin may decrease serum thyroxine and free thyroxine concentrations. 

Hyperthyroidism can be differentiated into overt and subclinical hyperthyroidism. Overt hyperthyroidism is diagnosed when the TSH level is suppressed, with free thyroxine (T4) and/or tri-iodothyronine (T3) levels above the normal reference range, in a person with symptoms of hyperthyroidism. Subclinical hyperthyroidism is diagnosed when the TSH level is suppressed, with free T4 and T3 levels within the normal reference range. The prevalence of overt hyperthyroidism is about 20 per 1000 women and 2 per 1000 men (including previously treated cases) with the annual incidence of overt hyperthyroidism is about 1 per 1000 women and is negligible for men. The prevalence of subclinical hyperthyroidism is 2% in adults, and 3% in those older than 80 years. 

Three main modalities of therapy should be considered for patients with thyrotoxicosis, namely, medical therapy, surgical thyroidectomy, and radioiodine. The choice between these therapies should be dictated by the clinical nature of the disease, the patient s general health, her desire for pregnancy or need to care for young children, and overall patient preference. Treatment is initially monitored by free thyroxine (T4) values, as suppression of thyroid-stimulating hormone (TSH) may persist for months despite adequate management. 

Answer The results of the laboratory tests confirm the hypothyroxinemia (i.e., low serum total T4 concentration). The calculated free thyroxine index... 

Thyroid effect. Goffee oil, administered orally to 11 healthy normolipemic volunteers at a dose of 2 g/day for 3 weeks, produced no effect on serum total and free thyroxine, triiodothyronine, and thyroid-stimulating hormone... 

TSH, T3, T, T3-resin uptake, free thyroxin index), but careful attention to early signs and symptoms of hypofunction (e.g., weight gain, cold intolerance, hair loss) may be more productive. Thyroid-stimulating hormone is sensitive to early thyroid changes and, if elevated, should prompt treatment with thyroid supplements to avoid goiter or hypothyroidism. 

Her weight is 136 lb (61.8 kg), an increase of 10 lb (4.5 kg) in the last year. Her thyroid gland is not palpable and her reflexes are delayed. Laboratory findings include a thyroid-stimulating hormone (TSH) level of 14.9 pIU/mL and a free thyroxine level of 8 pmol/L. Evaluate the management of her past history of hyperthyroidism. Identify the available treatment options for control of her current thyroid status. 

The etiology and pathogenesis of hypothyroidism are outlined in Table 38-5. Hypothyroidism can occur with or without thyroid enlargement (goiter). The laboratory diagnosis of hypothyroidism in the adult is easily made by the combination of a low free thyroxine and elevated serum TSH (Table 38-2). 

In postmenopausal women taking long-term mestranol there was a fall in the serum concentration of unbound (free) thyroxine, but it was not associated with hypothyroidism the serum concentration of thyroid-stimulating hormone was unchanged (29). 

Abdalla HI, Hart DM, Beastall GH. Reduced serum free thyroxine concentration in postmenopausal women receiving oestrogen treatment. BMJ (Clin Res Ed) 1984 288(6419) 754-5. 

Oral contraceptives can cause an increase in total thyroxine (163) and a fall in the percentage of free thyroxine (164). The uptake of radioactive iodine in the thyroid is usually normal total uptake of radioactive iodine may be reduced (164). The effect of progestogens on thyroxinebinding globulin may possibly counteract the estrogenic action. The net result will be a rise in protein-bound iodine and a fall in resin triiodothyronine uptake (165). It has been suggested that oral contraceptives may actually have some protective effect against thyroid disease. 

Iodine concentrations in breast milk and in random urine in neonates and the serum concentrations of neonatal TSH and free thyroxine on day 5 after delivery were measured after the use of povidone-iodine for disinfection after delivery (36). Iodine concentrations in the breast milk and neonatal TSH were significantly raised. Perinatal iodine exposure causes transient hypothyroidism in a significant number of neonates, in whom careful monitoring and follow-up of thyroid gland function are needed. It is better to avoid the use of iodine-containing antiseptics in pregnancy and neonates, especially if follow-up cannot be guaranteed. 

There was a modest rise in serum TSH concentration when rifampicin was given to a patient previously stabilized on thyroxine replacement (87). Rifampicin is believed to increase the metabolic clearance of both thyroxine and the inactive compound reverse triiodothyronine and in healthy volunteers it reduces circulating concentrations of total and free thyroxine, although in subjects without thyroid disease it has no effect on serum TSH (86). 

Despite the fact that she was clinically euthyroid, the authors suggested that this patient had amiodarone-induced hyperthyroidism. However, amiodarone inhibits the peripheral conversion of thyroxine to triiodothyronine it can therefore increase the serum thyroxine and suppress the serum TSH, as in this case. On the other hand, the reduced uptake by the thyroid gland is consistent with type 2 amiodarone-induced hyperthyroidism. The authors did not report the serum concentrations of free thyroxine and triiodothyronine. 

A 67-year-old man took amiodarone 200 mg/day for 20 months, after which it was withdrawn 8 months later his serum TSH was suppressed and the free thyroxine and free triiodothyronine were both raised there were no thyroid antibodies and an ultrasound scan showed a diffuse goiter with a nodule in the right lobe and reduced iodine uptake (45). Histological examination of the nodule showed a papillary cancer. 

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