Lipofuscin and abnormalities in colloid in the equine thyroid gland in relation to age

Lipofuscin accumulation and other histological changes in thyroid tissue, previously reported to be age-related, were studied in 31 horses aged up to 35 years. The number of lipofuscin granules relative to thyrocytes increased from birth to 5 years of age. There was a wide individual variation in the number of lipofuscin granules in thyrocytes in mature horses, but this was not directly related to age. Several abnormalities were identified in thyroid colloid. The prevalence of spherites, lipofuscin granules, nucleated cells and shreds of colloid increased with age, but the prevalence of calcium oxalate crystals, erythrocytes, basophilic zones and solid fragments of colloid did not. In horses younger than 7 years, particularly large lipofuscin granules were found in thyrocytes of a small proportion of follicles which also contained abnormal colloid. Such follicles became more common in older horses without being accompanied by large lipofuscin granules. No correlation was found between granule numbers and frequency of colloid abnormalities. These results cast doubt on the traditional assumption that lipofuscin is indigestible cellular residue, since there was little evidence for excretion of granules. It is postulated that lipofuscin in this tissue may be a normal stage in lysosomal catabolism.