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Measurements of Serum 3',5'-Diiodothyronine and 3,3'-Diiodothyronine Concentrations in Normal Subjects and in Patients with Thyroid and Nonthyroid Disease:Studies of 3',5'-Diiodothyronine Metabolism^*

JENS FABER, CARSTEN KIRKEGAARD, IB BO LUMHOLTZ, KAJ SIERSBÆK-NIELSEN and THORKILD FRIIS

Medical Department E, Frederiksberg Hospital, DK-2000 Copenhagen F, Denmark

A RIA of 3',5'-diiodothyronine (3',5'-T₂) in serumhas been established using small Sephadex G-25 (fine) columns. The lower detection limit was 0.3 ng/dl, which was sufficiently low to measure the concentration of 3',5'-T₂ in all sera except three from hypothyroid patients (n = 13). The mean (±SD) serum 3',5'-T₂ concentration in 34 euthyroid healthy subjects was 3.9 ± 1.5 ng/dl. No difference was found between males and females, and the concentration was not influenced by age. The concentration in 27 hyperthyroid patients was 14.4 ± 5.6 ng/dl, and in 13 hypothyroid patients was 1.3 ± 1.0 ng/dl. Both values differed significantly from that found in controls (P < 0.01). Normal values were found in patients suffering from liver cirrhosis (n = 12), uremia (n = 11), disseminated malignancies (n = 13), and acute myocardial infarction (n = 13). Administration of propylthiouracil (n = 15), but not carbimazole (n = 8), to L-T₄-substituted hypothyroid patients resulted in an increase of serum 3',5'-T₂ from 2.7 ± 1.1 to 3.8 ± 1.6 ng/dl (P < 0.01).

Using a similar method serum 3,3'-T₂ also was measured. The lower detection limit was 0.2 ng/dl, sufficiently low to measure the concentration in all sera except five from hypothyroid patients (n = 13). The mean (±SD) serum 3,3'-T₂ concentration in the control group was 1.9 ± 0.8 ng/dl. No difference was found between males and females, while serum 3,3'-T₂ decreased slightly with age (P < 0.05). The hyperthyroid group had elevated levels (7.8 ± 4.2 ng/dl), while the hypothyroid group showed decreased values (0.5 ± 0.4 ng/dl). Both values differed significantly from that found in controls (P < 0.01). Normal levels were found in the patients with liver cirrhosis, while significantly decreased values were found in the patients with uremia (0.5 ± 0.3 ng/dl), disseminated malignancies (1.1 ± 0.5 ng/dl), and acute myocardial infarction (1.2 ± 0.6 ng/dl). The administration of propylthiouracil, but not of carbimazole, led to a decrease in serum 3,3'-T₂ from 2.2 ± 0.6 to 1.8 ± 0.6 ng/dl (P < 0.05).

The extrathyroidal metabolism of 3',5'-T₂ was studied in five L-T₄-substituted hypothyroid patients using a noncompartmental method. Radioactive 3',5'-T₂ was separated from radioactive iodine, iodoproteins, and 3'-T₁ by means of gel and antibody separation. The mean metabolic clearance rate was 299 liters/ day-70 kg (range, 190-459 liters/day · 70 kg). The mean 3',5'-T₂ production rate was 12.6 µg/day-70 kg; the mean half-life time in serum was 10.6 h. (J Clin Endocrinol Metab 48: 611, 1979)

^* This work was supported by the Danish Hospital Foundation for Medical Research, Region of Copenhagen, The Faroe Islands, and Greenland and the Danish Medical Research Council.

To whom all correspondence and requests for reprints should be addressed.

Received April 18, 1978.

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