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Hypothyroidism is the most common
endocrine disease in dogs.
Autoimmune thyroiditis is the most common autoimmune disease
in dogs, and results in 80% or more cases of hypothyroidism.
Hypothyroidism is very over-diagnosed.
Hypothyroidism is very under-diagnosed.
How can all of these
statements be true? Confused? It would seem you are from the
contents of my in-box, and quite possibly your vet it too.
Thyroid hormones act
throughout the body to regulate metabolism, but they act at
the cellular level. While it is true that the hormones are
transported in the blood, the levels measured in the blood
may not accurately reflect the levels reaching the cells.
The body’s needs for thyroid hormone vary almost constantly,
and the production and release of hormones as well as their
uptake as needed is under exquisite control. The levels we
measure in the blood represent a snapshot in time, and may
not give us accurate data over the long-term.
What thyroid hormone does.
There are two thyroid
hormones, levothyroxine (T4) and triiodothyronine (T3) so
called because they have respectively 4 and 3 atoms of
iodine per molecule. The thyroid gland manufactures both
hormones from iodine and the amino acid tyrosine. While the
gland manufactures 80% T4 and only 20% T3, and this is
reflected in the levels of hormone measured in the blood,
only T3 actually performs a physiological role in the cell.
Most of the hormones are transported in the blood attached
to proteins and only a small amount is unbound or free. To
enter cells though the hormone must rid itself of the
protein. Once inside the cell any T4 is converted to T3. It
is the role of T3 which we will now consider.
One of the most important
functions of thyroid hormone is to help convert the basic
elements of food stuffs – carbohydrates, fats and protein -
into energy and heat. T3 is very active in the mitochondria
of all cells increasing energy production and increasing the
metabolic rate of the cell. It will increase the synthesis
of proteins by increasing gene transcription rate. Increased
cellular activity results in an increase in oxygen
requirements, and so T3 increases heart rate to help meet
these needs. Increased metabolism in the cells requires
fuel, and so increased levels of T3 result in weight loss,
unless the body takes in more calories. Increased work
raises body temperature at the cellular level. Food intake
and external temperature affect thyroid hormone production.
Levels will increase in the cold to help warm the body, they
will increase if the animal eats more. In starvation
production of thyroid hormones decreases as it does in hot
weather. Reducing thyroid hormone levels not only burns
fewer calories in starvation but also reduces ketone body
production. Ketones are produced when the body starts to use
fat rather than carbohydrates for energy and an excess can
damage the kidneys.
Thyroid stimulating
hormone as well as increasing production and release of
thyroid hormones from the thyroid, stimulates brown fat
production. This fat is rich in mitochondria, and under the
influence of T3 increases body heat without muscle activity.
Slow healing injuries and some chronic illnesses lower
levels of thyroid hormone, sick euthyroid syndrome, but they
will rebound once the injury is healed or the body healthy
again.
Thyroid hormones are
particularly important during development. T3 binds to
special receptors and regulates the expression of genes,
increasing production of some proteins and reducing the
production of others within the nucleus of the cell. Not all
genes are under thyroid control, and the affected genes may
be different for different species. For example only in dogs
are seizures associated with hypothyroidism, whereas in all
species fatigue, apathy, obesity in the face of reduced
appetite and heat seeking are. In adults, thyroid hormones
will also influence the rate at which dying cells are
replaced and the ability of the body to heal itself.
In the brain and
peripheral nervous system, T3 affects the production of and
sensitivity to neurotransmitters such as serotonin,
dopamine, epinephrine and norepinephrine. Reduced levels can
seriously impact behavior, as well as causing lethargy,
depressed neurological function, slow muscle and nerve
response and reduced muscle tone. The gut’s motility will be
reduced, leading to constipation
Thyroid hormone levels are regulated by and interact with
other hormones. The hypothalamus reacts to a reduction in
circulating thyroid hormone levels by increasing production
and release of thyrotrophin releasing factor (TRF). This in
turn causes the anterior pituitary gland to increase
production and release of thyroid stimulating hormone (TSH)
a.k.a. thyrotrophin, which in turn acts on tissue in the
thyroid gland to increase production and release of thyroid
hormones. Estrogen partially blocks the efficacy of thyroid
hormones, and so intact females tend to have reduced levels
of thyroid hormones when in season. Thyroid is also needed
for normal gonadal function, and infertility can result from
hypothyroidism. Thyroid hormone also boosts milk production
in lactating mammals. The role of growth hormone is more
complex; it partially blocks the efficacy of T3, but also
complements its effects on growth, development and
metabolism. Young animals have significantly higher
metabolic needs, and should have higher levels of thyroid
hormones than older animals until they are 18-24 months.
(Small breeds have relatively higher metabolic rates, and
large and giant ones lower metabolic rates, and again this
should be reflected in their normal levels of circulating
hormones. Low thyroid levels delay cortisol clearance and
this often results in a state of prolonged stress.
Signs of hypothyroidism
Hypothyroidism comes on
gradually. In older animals it is often mistaken for aging,
and owners expect their dogs to become calmer and less
active with age. In normal beardies this is really not that
apparent, and I would always test the thyroid in any beardie
that seemed to be aging especially if it was younger than 10
or 12 and had no painful condition that might be expected to
limit its activity levels.
Because thyroid hormone
affects every cell in the body the signs can be extremely
varied. In the early stages behavioral problems are usually
the first indication of a problem, and these can occur when
levels of thyroid hormones in the blood are in the low
normal, suboptimal or borderline range – remembering that
young dogs should have higher levels. In young dogs,
hypothyroidism can present as attention
deficit/hyperactivity disorder – the pup or young dog has
trouble sitting still and is slow to learn, failing to
retain information. These dogs may also be antisocial, asked
to leave training classes, and appear to bounce off the
walls. Often, despite the characteristic idea we have of a
hypothyroid dog, they are skinny and not obese. They may
however show one of more of the non specific signs of
hypothyroidism listed below. These may also show up in older
dogs and may be the major presenting sign until you dig
deeper and establish a pattern suggestive of poor thyroid
function.
____Excessive shedding,
patchy hair loss or “rat tail”
____Dry skin or dull, dry hair coat
____Recurrent infections (especially ear, skin and foot
infections)
____Tendency to gain weight
____Heat seeking behavior (acts cold)
____Increased sleep time
____Hyperactivity
____Slow learning
____Seizures
____Worried look, tragic facial expression or looking “old”
____Reduced hearing, sight and scenting ability
____Chronic gastrointestinal symptoms
____Loss of muscle or bladder tone
____Head tilt
____Change in character of bark
____Exercise intolerance
____Infertility, false pregnancy or weak, dying or stillborn
puppies
____Recurring eye infections
____Premature graying
Because of the
constant state of stress in which hypothyroid dogs live as
well as the changes in the central neurotransmitter levels
which affect behavior, I see many hypothyroid dogs present
as behavioral cases. The actual problem behaviors tend to be
more of a reflection of how a dog has learned to handle
stress and can fall into any of the three main behavioral
problem categories. Aggression can be owner directed, fear,
interdog or guarding. It is rarely territorial and this is
usually treated separately if there is comorbidity. Fears
and phobias of all types can be seen including fear of
thunderstorms and other loud noises, which seem to be common
in beardies. Obsessive compulsive and repetitive behaviors
are also frequently seen. In beardies a favorite seems to be
seeing ghosts or staring at blank walls. In other breeds
you’ll more often see tail chasing, light fixations and
obsessive ball play.
Diagnosis
And here comes the
rub. Thyroiditis is progressive, the body can maintain
normal serum thyroid levels or near normal ones if there is
no increase in hormonal needs with only 60% of the gland
functioning. Elevated levels of circulating autoantibodies
to T3 or T4 or to thyroglobulin - the parent protein to
which T3, T4 and their precursors are bound in the thyroid
gland – is always indicative of thyroiditis. Because the
blood is monitored as a snapshot in time though, they are
not constantly elevated and may well be missed. The
pituitary gland may increase production of TSH (and the
hypothalamus of TRF), but ultimately it becomes depleted and
so cTSH may not always be elevated in hypothyroid dogs. The
body tries to protect T3 levels and these tend to be less
affected by sickness than T4. Elevated serum levels of
cholesterol in the dog usually indicate hypothyroidism,
although disease of the liver and other causes do exist for
this. It’s not a perfect science though, and in diagnosing
hypothyroidism you cannot rely solely on the numbers from a
particular test. Only dogs showing signs of thyroid related
illness should be treated except in the case of elevated
autoantibody levels. In other dogs, underlying disease or
injury should be sought out.
Replacing the
body’s hormones with synthetic ones at the recommended
dosage (0.1mg/12-15 lbs body weight twice a day, usually 0.3
mg (bitch) or 0.4 mg (dog) twice a day in beardies) will not
induce hyperthyroidism. Dogs absorb thyroid hormones very
poorly (which is why they should be given 20 minutes before
meals), and except in sighthounds this level is perfectly
safe. The synthetic hormone will cause the dog’s own thyroid
to reduce hormone production – although if thyroid tissue
still remains in should be able to respond to the dog’s
subtly changing needs for the hormone. This will not harm
the remaining thyroid tissue, and will not speed up
deterioration, although it probably won’t stop this
continuing either. If there is absolutely no change in the
dog’s (apparently) thyroid related issues (I usually trial
for at least 2 months depending upon the problem), the dog
can be gradually weaned off thyroid hormones over 2 or 3
weeks allowing its own gland to increase production of
thyroid hormones (if it is capable of doing so).
It can be easy to
overlook the signs of hypothyroidism as they creep in
insidiously, but in terms of quality of life, addressing
thyroid issues can be immensely rewarding for both the dog
and the quality of the bond it has with its family. Yes, it
can be over-diagnosed if a dog is sick for other reasons and
under-diagnosed especially in the early stages if we aren’t
looking for subtle changes. For a small gland though it
surely has a mighty impact on our dogs’ lives and on our own
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