Clinicopathology of Thyroid

Gland Anomalies in Chelonians
- by J. GÁL -

The thyroid gland is situated outside of the celom (body or somatic) cavity, in front of the basis cordis (the base of the heart) in chelonians. Its size generally varies between that of a millet seed and a lentil
(Figure 1), but may reach that of a cherry pit in large-bodied tortoises (e.g., Geochelone spp.). A healthy thyroid gland usually has a brownish red color, with a homogeneous parenchyma and a dense but flexible touch (IPPEN et al. 1985, FRYE 1991). From the outside the organ is enclosed in a capsule of subcutaneous tissue. Its parenchyma consists of so-called acini, the cavity of which has a circular
or ovoid shape in histological preparations (Figure 2). In turn the cavity of the acini is lined with cuboidal type epithelium cells, the morphology of which varies according to the functionality level of the
gland (Figure 3). In case the acini are filled to a great extent with colloid material, the epithelium cells are flattened whereas in case of reduced colloid contents they are high and resemble cuboidal-type
cells. Normally the epithelium cells are aligned in a single row, and their nuclei are central or basal.

The thyroid gland of goiterous animals becomes considerably enlarged and distorted. In such
instances hormone levels are also greatly reduced in comparison with healthy individuals. Clinical
symptoms develop as a result of an altered production of hormones coupled with an increase in size of
the thryoid gland. Goiterous chelonians lose their appetite, take less food with less intensity in the initial, and completely refuse eating in the ultimate phase. Their activity levels decrease, and they move
around less compared with their healthy conspecifics. At the same time their body mass increases as a result of fluid accumulation (edema) in their organs and tissues. Occasionally, the thyroid gland
becomes enlarged to such an extent that when the animal extends its neck, the protruding organ can be
seen on the ventral surface of the neck beside the foreleg (Figure 5A).

Diagnosing goiter is a difficult task and various factors need to be taken into consideration. In case
herbivorous species are known to have been fed goitrogenous food (e.g., cruciferous vegetables) goiter
should be suspected. Very few references exist as to the blood levels of thyroid hormones. However,
determining hormone levels may still be a usefool diagnostic tool. According to MCARTHUR et al.
(2004) values less than 0.29 mg/dl T4 and 4.5 mg/dl T3, respectively, point to iodine metabolism disorders.

Upon dissection an accumulation of fluid can be observed in the somatic cavity. Fluid accumulation in the subcutaneous tissue is another characteristic sign, seen as a thickening of the legs and neck.
Occasionally, eyelids are slightly swollen. A fatty degeneration of the liver can also be observed in dissected chelonians. TOPPER et al. (1994) reported hemoglobin-caused kidney failure in addition to
fatty liver degeneration in an eastern diamondback rattlesnake. The thyroid gland enlarges in all
instances and can occasionally take an irregular shape (Figure 5B). Dilated acini can sometimes be
seen on the surface of the organ even with the naked eye. An enlarged thyroid gland is often lighter in
color than a normal one. Blood-vessels running in the capsula of the thyroid gland can often be substantially dilated (capsular congestion). The cutting surface is frequently moist and shiny, and the
irregular sized acini can be detected by sight. In histological preparations the irregular and enlarged
acini are the most obvious (Figures 6–7). The epithelium cells lining the acini are polymorphic,
becoming cuboid-like in certain, and considerably flattened in other areas. In a number of cases coral-
like epithelium horns emerging from the acinus cavities can be observed. Staining characteristics of the
colloid material may also alter.

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