Canine Hypothyroidism

Introduction

Hypothyroidism is the most common canine endocrine disorder; most cases are seen in dogs over one year of age although perhaps 10% may be in younger animals. A small number (~ 3%) of cases are congenital, while the rest relate to pathology acquired during growth.

The canine thyroid gland is located lateral to the trachea, in the region of the proximal tracheal rings (1). Histologically, the functional unit is the thyroid follicle, which comprises the follicular cells (thyrocytes) and colloid, each gland containing both large (resting) follicles and small (active) follicles (Figure 1). The embryogenesis and maturation of the fetal thyroid axis can be divided into three periods:

- The first period corresponds to embryogenesis of the gland; the thyroid forms in the third and fourth pharyngeal pouches, beneath what will become the tongue. Migration of the thyroid towards the trachea and the start of its physiological function are regulated by transcription factors. TSH-R (thyroid stimulating hormone (TSH) receptor) is expressed at the end of embryonic development of the gland, with the initiation of iodine uptake in the fetal thyroid gland (2). During this stage 100% of the thyroxin (T4) originates from the mother, and is essential for correct development of the nervous system and other embryonic tissues.

- The second period corresponds to maturation of the fetal thyroid axis and commences when the thyroid reaches its anatomical location. At this point the hypothalamus begins to release thyrotrophin-releasing hormone (TRH), and consecutively TSH synthesis and release, iodine uptake by the fetal thyroid gland, and T4 synthesis is observed. Note that a large proportion of T4 undergoes deionization in the fetus to the reverse form of triiodothyronine (rT3). During this period 50% of T4 is maternally derived, but this gradually decreases to 20% at the end of the gestational period. This maternal contribution is essential for fetal growth and the maintenance of euthyroid status in the newborn puppy during the first 24h of life. [...]