Front Page : Library : The Effects of Thyroid Hormones...

BACKGROUND: Hypothyroidism is the most common cause of endocrine-induced alopecia in dogs. Deficiency of thyroid hormones arrests hair growth and leads more hair follicles to go into telogen (resting, detaching phase). Detachment of hair within the follicle does not lead to hair loss by gravity. The orientation of the roof shingle-like cuticle of the hair shaft and the slanted position of hair follicles aid in retaining detached hair until sufficient friction occurs to remove the hair. Clinical alopecia from hypothyroidism then occurs wherever the hair cycle is the most rapid and the friction is greatest to the body, i.e. end of the tail, pinna, and lateral thighs. The pattern of hair loss may not be symmetrical at first nor occur in the same sequence in all individuals. An experimental model for hypothyroidism might provide the opportunity for closer and longer observation of the effects of hypothyroidism on the skin and hair coat of dogs.

SUMMARY: Skin biopsies were performed on three groups of three male beagles each: euthyroid and radioiodine-induced hypothyroid without thyroid supplementation and with thyroid supplementation (0.1 mg/ 10 lb, once daily, orally). All groups were observed for 10 months. Every two months, serum thyroid hormone levels were measured and one week before skin biopsy was performed at 10 months. Skin samples taken from clipped sites on the dorsal right and left side of the thoracic midline. Examinations included blinded morphologic and morphometric determinations from five follicular units. The stage of the hair growth cycle was also determined. Two months after clipping, there was slower regrowth of hair in untreated hypothyroid dogs. Microscopically, these dogs had more hair in telogen and less hair shafts in follicles than control dogs or treated hypothyroid dogs. Treated hypothyroid dogs had more hairs in anagen (growth phase) than control dogs. The authors concluded that the beagles in this study retain telogenic hair for long periods and that thyroid hormone replacement stimulates anagen beyond the level of control dogs.

CLINICAL IMPACT: The findings of this well-controlled study confirm many clinical observations such as thyroid hormone supplementation can (temporarily) induce hair growth by stimulation and synchronization of the hair cycle in treated hypothyroid (or euthyroid) dogs. Temporary or partial growth of hair in alopecic areas after treating with thyroid hormone at replacement doses does not validate that the diagnosis of hypothyroidism was correct. Complete and permanent regrowth may, if alopecia occurs again with hormone replacement withdrawal.
This study also demonstrated that without traction on telogenic hair, alopecia develops slowly or not at all in dogs with arrested hair growth. It is no surprise to anyone who has carried experimental dogs that are not groomed daily that a lot of telogenic hair is retained until the friction of a lab coat is rubbed on their hair coat. Had these experimental dogs been groomed daily or been permitted to run through vegetation or otherwise had the hair coat frictional opportunities of a typical companion or hunting dog, it is likely clinical alopecia would have been more prominent in the untreated hypothyroid dogs. A third finding consistent with clinical observations is that the dose of 0.01 mg/lb/day of levothyroxine will meet or exceed normal serum thyroid hormone levels and result in normal hair growth in hypothyroid dogs

Front Page : Library : The Effects of Thyroid Hormones...