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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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