chapter 28 Hypothyroidism......................................
DEFINITION/OVERVIEW
Decreased production of thyroid hormones (tetraiodothyronine – T 4 ; triiodothyro-
nine – T 3 ) by the thyroid gland.
Thyroid hormones are essential for normal growth and development: inadequate thy-
roid hormone affects many metabolic processes and nearly every organ system.
Thyroid hormones are necessary for the normal maturation of skin and initiation of
the anagen phase of the follicle cycle.
Hypothyroidism is the most common endocrine disorder in dogs; hypothyroidism is
also the most overdiagnosed endocrinopathy in dogs due to the lack of a single readily
available and reliable test for hypothyroidism.Clinical signs of hypothyroidism mimic those of other causes of alopecia.
Dermatologic signs include lack of hair coat growth, dull dry brittle coat, initial alope-
cia in areas of wear (bridge of nose, elbows, tail), becoming more widespread along
the entire trunk in a symmetric distribution, myxedema, hyperpigmentation, sebor-
rhea, and recurrent bacterial folliculitis.Treatment is by thyroid hormone (T 4 ) supplementation.
ETIOLOGY/PATHOPHYSIOLOGY
Thyrotropin (TSH) released by the pituitary gland is the major modulator of hormone
secretion by the thyroid gland; T 3 inhibits TSH secretion.
Effect of thyrotropin-releasing hormone from the hypothalamus on overall activity of
the thyroid gland unclear; tertiary hypothyroidism not documented in dogs or cats.
Thyroid hormones in plasma primarily protein bound; less than 1% is “free” in cir-
culation; pituitary-thyroid axis functions to maintain free T 4 (fT 4 ) levels.
T 3 is the metabolically active hormone; T 4 is primarily a prohormone.
T 4 : over 80% of secreted hormone; biologic activity upregulated by conversion to the
active form (T 3 ) in peripheral tissues; conversion to reverse T 3 (rT 3 ) reduces activity.
Only unbound hormone penetrates cells and has biologic activity.
Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Small Animal Dermatology, Third Edition.
Karen Helton Rhodes and Alexander H. Werner.
©2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.
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