Comparison of colloid, thyroid follicular epithelium, and thyroid hormone concentrations in healthy and severely sick dogs

Objectives - To compare serum concentrations of total thyroxine (TT₄), free thyroxine (fT₄), and thyroid-stimulating hormone (TSH), as well as measures of thyroid follicular colloid and epithelium, between groups of healthy dogs and severely sick dogs. Design - Cross-sectional study. Animals - 61 healthy dogs and 66 severely sick dogs. Procedure - Serum samples were obtained before euthanasia, and both thyroid lobes were removed immediately after euthanasia. Morphometric analyses were performed on each lobe, and serum TT₄, fT₄, and TSH concentrations were measured. Results - In the sick group, serum TT₄ and fT₄ concentrations were less than reference range values in 39 (59%) and 21 (32%) dogs, respectively; only 5 (8%) dogs had high TSH concentrations. Mean serum TT₄ and fT₄ concentrations were significantly lower in the sick group, compared with the healthy group. In the healthy group, a significant negative correlation was found between volume percentage of colloid and TT₄ or fT₄ concentrations, and a significant positive correlation was found between volume percentage of follicular epithelium and TT₄ or fT₄ concentrations. A significant negative correlation was observed between volume percentages of colloid and follicular epithelium in both groups. Conclusions and Clinical Relevance - TT₄ and fT₄ concentrations are frequently less than reference range values in severely sick dogs. Therefore, thyroid status should not be evaluated during severe illness. The absence of any significant differences in mean volume percentages of follicular epithelium between healthy and severely sick dogs suggests that these 2 groups had similar potential for synthesizing and secreting thyroid hormones.