Serum Thyroid Hormone and Thyrotropin Concentrations During Thyroxine and Triiodothyronine Therapy

Abstract

Measurements of serum triiodothyronine (T 3 ), thyroxine (T 4 ) and thyrotropin (TSH) concentrations were made in hypothyroid patients at 2-hr intervals for 48 hr before and during chronic therapy with single daily doses of 25 and 50 fig T 3 or 100 and 200 ^g T 4 . No time-related changes were found in serum T 3 , T 4 or TSH levels during therapy with 100 or 200 fj.g T 4 daily. In patients treated with T 3 , prompt increases in serum T 3 levels occurred which reached a peak after 4 hr and then gradually returned to base line. Base line serum T 3 levels were, respectively, 101 ± 13 and 94 ± 9 ng/100 ml (SEM) and the peak levels were 223 ± 17 and 235 ± 18 ng/100 ml on the 2 study days in patients receiving 25 fig T 3 daily. Base line serum T 3 levels were 109 ± 18 and 116 ± 13 ng/ 100 ml and peak values were 350 ± 30 and 375 ± 31 in the patients receiving 50 fig T 3 daily. Serum TSH levels were elevated (mean basal serum TSH 13.1 ± 3.9 and 14.2 ± 4.7 /iU/ml) in the patients receiving 25 fxg T 3 daily and also elevated (mean basal serum TSH 11.8 ± 5.4 and 11.8 ± 6.0 /iU/ml) in the patients receiving 50 (xg T 3 daily. No changes in serum TSH were observed in the intervals between doses of T 3 in either group.These results indicate that once daily T 4 therapy is not associated with any acute changes in either serum T 4 , T 3 or TSH levels. While striking alterations in serum T 3 levels follow once daily T 3 therapy, these changes are not associated with any acute changes in serum TSH levels. (7 Clin Endocrinol Metab 39: 923, 1974)

Acute Effects of Triiodothyronine (T3) Replacement Therapy in Patients with Chronic Heart Failure and Low-T3Syndrome: A Randomized, Placebo-Controlled Study

“…The protocol we adopted for synthetic L-T 3 administration offers two main advantages: 1) constant infusion of substitutive doses of L-T 3 makes it possible to rapidly establish stable T 3 levels within the physiological range (Fig. 1), which is at variance with multiple bolus injections or an oral regimen; and 2) constant infusion of substitutive doses of L-T 3 is more effective in promoting nuclear action of T 3 and T 3 -mediated transcription in the myocardium when compared with multiple bolus injections (14) or to an oral regimen (17)(18)(19). However, previous studies on euthyroid patients with DC showed that short-and medium-term treatment with 0.1 mg/d synthetic L-T 4 increased CO and reduced SVR in the absence of significant changes in HR and catecholamine circulating levels (16).…”

Effect of serum triiodothyronine on regulation of cardiac gene expression: role of histone acetylation

“…Multiple tissues, including the myocardium, do not convert T 4 to T 3 and must depend on T 3 derived from the serum for cellular actions and regulation of gene transcription (25). Daily administration of T 3 either by oral ingestion or by parenteral injection produces wide variations in both serum T 3 levels and the rate of T 3 -mediated transcription in the myocardium (8,22). The pharmacokinetics of T 3 administration on serum T 3 in hypothyroid patients were studied by Saberi and Utiger (22) by taking measurements every 2 h after once-daily oral therapy of 25 or 50 g of T 3 .…”

Thyrotropin-induced hyperthyroidism caused by selective pituitary resistance to thyroid hormone. A new syndrome of "inappropriate secretion of TSH".

“…2). The serum T8 was probably much higher than this minimal level through most of the day (25). The thyroidal response to the lowered circulating TSH concentration was manifested by a decline in the uptake of radioiodine at 4 h from 26 (26).…”