I copied this from the BCA health website...

Seasonal Flank Alopecia
Flank alopecia is a frustrating but generally harmless loss of hair
over the ribs, flanks and haunches of affected dogs. The exposed skin
often becomes very dark and thickens and may develop a polished
appearance. Conversely, flaking, scaling and dryness may also be
present. Until recently, most cases were erroneously mistaken for a
symptom of hypothyroidism and dogs with this variety of hair loss
should be screened for thyroid disease. If the treating veterinarian
determines the dog to be essentially normal with regards to it's
endocrine system, then seasonal flank alopecia should be considered
as the likely cause of bilateral symmetrical hair loss across the
ribs, flanks and possibly hind quarters.
There is no universally accepted or effective treatment for SFA.
As it it rarely associated with discomfort or itching, it is more of
an aesthetic issue although measures should be taken to protect the
exposed skin from the elements, especially sunburn in the case of
light skinned dogs. Use of melatonin and/or zinc supplements have
been reported helpful anecdotally.
As the name implies, the condition is seasonal, most often
beginning in mid to late winter. This has led to the hypothesis that
it is triggered by a lack of exposure to adequate amounts of natural
light. This has led some owners to provide artificial full spectrum
light in indoor areas wherein the affected animals spend
considerable amounts of time. Anectdoctal evidence suggests this may
decrease at least the severity of hair loss. The condition will
typically begin to resolve late spring and improve of disappear by
mid summer but will often recur the next winter