Serum Thyroid Hormone and Thyrotropin Concentrations During Thyroxine and Triiodothyronine Therapy
Abstract
Measurements of serum triiodothyronine (T3), thyroxine (T4) 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 μg T3 or 100 and 200 μg T4. No time-related changes were found in serum T3, T4 or TSH levels during therapy with 100 or 200 μg T4 daily. In patients treated with T3, prompt increases in serum T3 levels occurred which reached a peak after 4 hr and then gradually returned to base line. Base line serum T3 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 μg T3 daily. Base line serum T3 levels were 109 ± 18 and 116 ± 13 ng/100 ml and peak values were 350 ± 30 and 375 ± 31 in the patients receiving 50 μg T3 daily. Serum TSH levels were elevated (mean basal serum TSH 13.1 ± 3.9 and 14.2 ± 4.7 μU/ml) in the patients receiving 25 μg T3 daily and also elevated (mean basal serum TSH 11.8 ± 5.4 and 11.8 ± 6.0 μU/ml) in the patients receiving 50 (μg T3 daily. No changes in serum TSH were observed in the intervals between doses of T3 in either group. These results indicate that once daily T4 therapy is not associated with any acute changes in either serum T4, T3 or TSH levels. While striking alterations in serum T3 levels follow once daily T3 therapy, these changes are not associated with any acute changes in serum TSH levels.
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LT4 has a half-life of approximately 7 days.
“The long (approximately 7 day) half-life allows once-a-day dosing, and stable serum levels of both T4 and T3 (36).”
From Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014) · cites this paper as [36]
Once-daily LT4 dosing yields stable serum T4 and T3 levels.
“The long (approximately 7 day) half-life allows once-a-day dosing, and stable serum levels of both T4 and T3 (36).”
From Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014) · cites this paper as [36]
In 16 hypothyroid patients during chronic treatment with LT3 (25 or 50 mcg/day), at steady state, LT3 administration resulted in acute elevation of serum T3 about 3 hours after the dose to above the upper limit of the reference range.
“The study of 16 hypothyroid patients (21–76 years old; 14 females) during chronic treatment with LT3 (25 or 50 mcg/day) shows that, at steady state, the administration of LT3 resulted in an acute elevation of serum T3 ∼ 3 hours after the dose, to above the upper limit of the reference range”
From Liothyronine and Desiccated Thyroid Extract in the Treatment of Hypothyroidism (2020) · cites this paper as [52]
Serum T3 levels did not fluctuate in patients treated with LT4 as they did with patients treated with DTE.
“serum T3 levels did not fluctuate in patients treated with LT4 as it did with patients treated with DTE (52,89,90).”
From Liothyronine and Desiccated Thyroid Extract in the Treatment of Hypothyroidism (2020) · cites this paper as [52]
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“The available data relative to the pharmacokinetics of LT3 are contradictory, indicating half-life values ranging from 6 to 22 hours (36,543).”
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- Authors
- SABERI, M., & UTIGER, R. D.
- Affiliations
- University of Pennsylvania (US)
- Venue
- The Journal of Clinical Endocrinology & Metabolism
- Publisher
- The Endocrine Society
- Published
- 1974-11
- Volume
- 39
- Issue
- 5
- Pages
- 923-927
- ISSN
- 0021-972X (ISSN-L)
- 1945-7197
- OpenAlex
- W2140522098
- Cited by
- 79
- References
- 0
- Citations by year
- 2025: 1 · 2022: 2 · 2020: 2 · 2019: 2 · 2018: 1 · 2016: 2 · 2014: 2
- FWCI
- 5.80
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- The Journal of Clinical Endocrinology & Metabolism (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Health Sciences › Medicine › Endocrinology, Diabetes and Metabolism › Thyroid Disorders and Treatments
- Health Sciences › Medicine › Surgery › Thyroid and Parathyroid Surgery
- Health Sciences › Medicine › Endocrinology, Diabetes and Metabolism › Thyroid Cancer Diagnosis and Treatment
- Keywords
- hypothyroidism, triiodothyronine, thyroxine, thyroid-stimulating hormone, serum thyroid hormones, once-daily dosing, thyroid hormone replacement, dose-response relationship
- MeSH terms
- Adult, Aged, Female, Humans, Hypothyroidism, Male, Middle Aged, Thyrotropin, Thyroxine, Time Factors, Triiodothyronine
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