Kris­tel-Marie Ram­nath

Thy­roid dis­ease is a rel­a­tive­ly com­mon prob­lem in dogs. While it is usu­al­ly treat­able, learn­ing that your dog has a thy­roid con­di­tion is un­der­stand­ably con­cern­ing.

The thy­roid is a but­ter­fly-shaped gland that is lo­cat­ed on both sides of a dog’s neck. It pro­duces the hor­mone thy­rox­in (T4), along with sev­er­al oth­er im­por­tant thy­roid hor­mones. These hor­mones play a large role in a dog’s me­tab­o­lism and can cause ma­jor prob­lems when they are not pro­duced at nor­mal lev­els. A dog’s body tem­per­a­ture, heart rate, and food util­i­sa­tion are all af­fect­ed by the lev­el of thy­roid hor­mone in the blood­stream. The thy­roid gland is con­trolled by the pi­tu­itary gland, which is lo­cat­ed at the base of the brain.

Hy­pothy­roidism oc­curs when the thy­roid gland does not pro­duce and se­crete enough thy­roid hor­mones. The two most com­mon caus­es of hy­pothy­roidism in dogs are lym­pho­cyt­ic thy­roidi­tis (in­flam­ma­tion of the thy­roid gland), and id­io­path­ic thy­roid gland at­ro­phy (de­gen­er­a­tion of the thy­roid gland). With lym­pho­cyt­ic thy­roidi­tis, the im­mune sys­tem no longer recog­nis­es the thy­roid as nor­mal, and at­tacks it. It is un­clear why this oc­curs; how­ev­er, it is a her­i­ta­ble trait mean­ing that ge­net­ics play a role in this con­di­tion.

In id­io­path­ic thy­roid gland at­ro­phy, nor­mal thy­roid tis­sue is re­placed by fat tis­sue. This con­di­tion is al­so poor­ly un­der­stood. These two caus­es of hy­pothy­roidism ac­count for more than 95 per cent of the cas­es in dogs. The oth­er five per cent are due to rare dis­eases, in­clud­ing can­cer of the thy­roid gland.

Hy­pothy­roidism is typ­i­cal­ly seen in mid­dle-aged to old­er dogs and oc­curs more com­mon­ly in medi­um to large breed dogs.

When the meta­bol­ic rate slows down, vir­tu­al­ly every or­gan in the body is af­fect­ed, so dogs with hy­pothy­roidism show signs in mul­ti­ple body sys­tems. The de­struc­tion of the thy­roid gland usu­al­ly pro­gress­es slow­ly, there­fore signs are of­ten grad­ual and easy to miss at first.

Symp­toms of hy­pothy­roidism in dogs may in­clude:

• Weight gain de­spite no change in ap­petite

• Re­duced ac­tiv­i­ty lev­el—the dog may be lethar­gic, list­less, tire eas­i­ly, and sleep a lot

• In­abil­i­ty to tol­er­ate cold – the dog may seek out warm places to lie

• Hair loss or thin­ning of fur – the hair be­comes dry and brit­tle, and the coat looks dull

• Hair does not re­grow af­ter it has been shaved

• Chron­ic skin and ear in­fec­tions

• Skin thick­en­ing and dark­en­ing, with in­creased pig­men­ta­tion re­sult­ing in black­ened skin

• Ab­nor­mal func­tion of nerves caus­ing non-painful lame­ness, drag­ging of feet, lack of co­or­di­na­tion, and a head tilt

• Dry eye due to lack of prop­er tear pro­duc­tion

Hy­pothy­roidism can be di­ag­nosed eas­i­ly with a spe­cialised blood test that mea­sures thy­roid hor­mones. The most com­mon screen­ing test is called a to­tal thy­rox­in lev­el (TT4) test, which de­ter­mines the dog’s main thy­roid hor­mone lev­el. A low hor­mone lev­el ac­com­pa­nied by clin­i­cal signs in the dog is in­dica­tive of hy­pothy­roidism. Many vet­eri­nar­i­ans will then run ad­di­tion­al blood tests, ei­ther a free T4 lev­el or a full thy­roid pan­el, to con­firm the di­ag­no­sis of hy­pothy­roidism.

While there is no cure for hy­pothy­roidism, dogs can be treat­ed with thy­roid hor­mone re­place­ment ther­a­py. Two drugs are FDA-ap­proved for re­place­ment ther­a­py for di­min­ished thy­roid func­tion in dogs: Thy­ro-tabs Ca­nine and Thy­roKare. Both are pre­scrip­tion drugs con­tain­ing levothy­rox­ine sodi­um as the ac­tive in­gre­di­ent. The tablets are avail­able in dif­fer­ent strengths, and dogs are gen­er­al­ly start­ed on a stan­dard dose of thy­roid re­place­ment hor­mone, based on the dog’s weight. Since the dog’s tol­er­ance of the thy­roid re­place­ment hor­mone may change over time, the dose may need to be pe­ri­od­i­cal­ly ad­just­ed as thy­roid med­ica­tion can be over­dosed. It is usu­al­ly nec­es­sary to retest thy­roid hor­mone lev­els every six months.

Your dog will need to stay on thy­roid re­place­ment hor­mone for life. Un­treat­ed hy­pothy­roidism can short­en your dog’s life span (as near­ly every or­gan in the body is af­fect­ed by thy­roid hor­mone and by the me­tab­o­lism) and can have a neg­a­tive ef­fect on your dog’s qual­i­ty of life. Dogs with hy­pothy­roidism can live nor­mal, healthy lives when the dis­ease is man­aged with med­ica­tion.