L-thyroxine therapy

Paul TL, Kerrigan J, Kelly AM, Braverman LE, Baran DT. Long-term L-thyroxine therapy is associated with decreased hip bone density in premenopausal women. JAMA 1988 259(21) 3137-41. 

Hiasa Y, Ishida T, Aihara T, Bando M, Nakai Y, Kataoka Y, Mori H. Acute myocardial infarction due to coronary spasm associated with L-thyroxine therapy. Clin Cardiol 1989 12(3) 161-3. 

Heijckmann AC, Huijberts MS, Geusens P, de Vries J, Menheere PP, Wolffenbuttel BH. Hip bone mineral density, bone turnover and risk of fracture in patients on long-term suppressive L-thyroxine therapy for differentiated thyroid carcinoma. Eur J Endocrinol 2005 153(l) 23-9. 

Carella C, Mazziotti G, Rotondi M, et al. Iodized salt improves the effectiveness of L-thyroxine therapy after surgery for nontoxic goitre A prospective and randomized study. Clin Endocrinol 2002 57 507-513. 

TABLE 3. Iodine concentration of goiters in children and adolescents before and during iodide therapy (100-150 ng daily) for 4-8 months and before and during L-thyroxine therapy (6 month)... 

Under L-thyroxine therapy the low IC decreased even further. The T /TBG ratio slightly increased. The fall in the mean TSH-levels was marginally more noticeable and the fraction of TSH-levels below 2,1 p,U/ml virtually equal with 86% (Tab. 4). 

Levothyroxine (L-thyroxine, T4) is the drug of choice for thyroid hormone replacement and suppressive therapy because it is chemically stable, relatively inexpensive, free of antigenicity, and has uniform potency however, any of the commercially available thyroid preparations can be used. Once a particular product is selected, therapeutic interchange is discouraged. 

Thyroxine is absorbed best in the duodenum and ileum absorption is modified by intraluminal factors such as food, drugs, gastric acidity, and intestinal flora. Oral bioavailability of current preparations of L-thyroxine averages 80% (Table 38-1). In contrast, T3 is almost completely absorbed (95%). T4 and T3 absorption appears not to be affected by mild hypothyroidism but may be impaired in severe myxedema with ileus. These factors are important in switching from oral to parenteral therapy. For parenteral use, the intravenous route is preferred for both hormones. 

Cytomel is the synthetic form of T-3/L-triiodothyronine and was a commonly known trade or brand name among athletes. T-3/L-triiodothyronine is used as a form of thyroid hormone therapy mostly in Europe. Most bodybuilders favored this drug over synthetic forms of T-4/L-thyroxine due to its vastly superior activity level. 

Escobar-Morreale HF, Botella-Carretero JI, Gomez-Bueno M, et al. Thyroid hormone replacement therapy in primary hypothyroidism a randomized trial comparing L-thyroxine plus liothyronine with L-thyroxine alone. Ann Intern Med. 2005 142 412-424. 

M. B. Gutiick and P. R. Larsen. Acute deficiency of thyroxine-binding globulin during l.nqnnagjnaae therapy. N. Engt. J. Med. 301 251 (1979). 

Bilberry extract 200 mg/(kg day) administered intraperitoneally to euthyroid rats increased radiolabeled triiodothyronine (T3) transport into the brain, compared to vehicle only (21). Postulated mechanisms include central or peripheral inhibition of L-thyroxine s (T4) deiodination to T3 inhibition of T3 protein binding or enhanced T3 binding to carrier proteins in the brain capillary wall (21). Whether bilberry could interact with thyroid replacement therapy remains to be seen. 

Iodine is readily absorbed when PVP-I is applied to the skin of a newborn infant, because of high cutaneous permeability, and neonates are very sensitive to iodine overload, as described previously. Topical PVP-I therapy is associated with a significant risk of hypothyroidism in neonates, especially very-low-weight babies (Smerdely et ai, 1989). Many cases of hypothyroidism induced by topical use of PVP-I have been reported in newborn infants, mainly from iodine-deficient regions (Markou et ai, 2001). However, a case of severe hypothyroidism in a neonate was also reported from North America, an iodine-sufficient region (Khashu et al. 2005). A premature infant developed severe hypothyroidism that required L-thyroxine replacement therapy after application of PVP-I for 20 days. 

In conclusion, the study indicates that the three types of therapy for endemic goitre are equally effective, provided the patients receive a sufficient quantity of iodine. The form of iodine administration, whether as iodide or as L-Thyroxine, is not important except for compliance, which is best when the patient takes only one tablet per day. 

A 62-year-old man with recurrent non-Hodgkin s lymphoma developed pulmonary tuberculosis, for which he received rifampicin. Within 2 weeks, his thyrotropin (TSH) concentration increased to 170 mU/1 and the serum concentrations of thyroxine (T4) and triiodothyronine (T3) fell to 24 pg/l and 180 ng/1 respectively. He was given thyroxine. After the course of rifampicin therapy had been completed, thyroxine was withdrawn and he remained euthyroid for 4 years. 

A 66-year-old woman with tuberculous peritonitis was given rifampicin and developed hypothyroidism (thyrotropin concentration 12.5 mU/1, T4 48 pg/l, T3 8.7 ng/1). She was given thyroxine for 3 months. Hypothyroidism developed again, and thyroxine was resumed for the duration of the course of rifampicin therapy and then withdrawn, after which she remained euthyroid for 42 months. 

---