3',5'-diiodothyronine release from perfused dog thyroids

Area of Science:

• Endocrinology
• Biochemistry
• Analytical Chemistry

Background:

• Thyroid hormone metabolism involves various iodothyronines beyond thyroxine (T4) and triiodothyronine (T3).
• The specific role and quantification of 3",5"-diiodothyronine (3",5"-T2) in thyroid physiology remain less understood.
• Accurate measurement methods are crucial for investigating the presence and secretion of less abundant thyroid hormones.

Purpose of the Study:

• To develop and validate a sensitive radioimmunoassay for quantifying 3",5"-T2.
• To measure the concentration of 3",5"-T2 in canine thyroid tissue and perfusate.
• To determine if the thyroid gland actively secretes 3",5"-T2 and in what amounts.

Main Methods:

• Development of a radioimmunoassay with a detection limit of 1-2 pmol/l and minimal cross-reactivity with T4, T3, and reverse T3 (rT3).
• Validation of the assay using dilution curves and assessment of potential artifactual generation or loss of 3",5"-T2 during sample processing.
• Measurement of 3",5"-T2 in thyroid hydrolysates and perfusate from canine thyroid lobes under basal and TSH-stimulated conditions.

Main Results:

• The radioimmunoassay demonstrated high sensitivity and specificity, with low coefficients of variation (5.1% intra-assay, 9.2% inter-assay).
• Minute amounts of 3",5"-T2 were detected in canine thyroid hydrolysate (1.66 ± 0.41 pmol/mg) and perfusate (basal: ~20 pmol/l; TSH-stimulated: ~170 pmol/l).
• Interference from T4 and rT3 was ruled out as a significant contributor to the measured 3",5"-T2 levels.

Conclusions:

• A reliable radioimmunoassay for 3",5"-T2 has been established.
• The canine thyroid gland contains and secretes 3",5"-T2, but in significantly lower quantities compared to other major thyroid hormones.
• Further research is warranted to elucidate the physiological significance of these small amounts of secreted 3",5"-T2.