Bioavailability of thyroid hormones from oral replacement preparations

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Bioavailability of thyroid hormones from oral replacement

preparations.

LeBoff

MS, Kaplan

MM, Silva

JE, Larsen

PR.

Abstract

We evaluated

gastrointestinal absorption in normal subjects of T4 and T3 from synthetic T3

tablets (Cytomel, SKF), desiccated thyroid tablets (Armour), thyroglobulin

tablets (Proloid, Warner-Chilcott) and synthetic L-T4 tablets (Synthroid, Flint

and Levothroid, Armour). Measurements of serum T4 and T3 concentrations and

free hormone indices were made at multiple times after tablet ingestion, and T3

content in tablets was measured by radioimmunoassay. The time to peak serum T3,

and the 26 hr intergrated increment in serum T3, Corrected for the amount if T3

ingested, were not significantly different for 75 micrograms of synthetic T3, 6

grains of desiccated thyroid (containing 99 micrograms T3) and 5 grains of

thyroglobulin (containing 90 micrograms T3), the mean integrated increment

values for the biological preparations being within 12% of those for synthetic

T3. The peak serum T4 concentration, the time to peak T4, and 48 hr integrated

increments in serum T4 and T3 were similar after 3 mg of Synthroid and

Levothroid. The mean peak serum Free T3 Index after 75 micrograms T3, 500, was

much higher than the mean peak Free T3 Index after 3 mg T4, 290. The time to

peak Free T3 Index was much less after 75 micrograms T3, 2 hr, than the time to

peak after 3 mg T4, 2 days. These results indicate that the time course and

extent of T3 absorption do not differ, whether the T3 is given as the synthetic

iodothyronine or as part of the thyroid protein, thyroglobulin. This approach appears

to be useful in determining bioavailability of thyroid hormones from oral

preparations and to assess the possibility of thyroid hormone malabsorption.

PMID: 7121261 [PubMed

- indexed for MEDLINE]