CONDITION
Hypothyroidism in Cats
Hypothyroidism in cats occurs when the thyroid gland produces less thyroid hormone than the body needs. This hormone influences how cells use energy, so when levels drop, many processes in the body can slow down. In cats, this condition is uncommon and tends to appear in specific contexts—most often after treatment for an overactive thyroid, or occasionally as a primary gland problem. Owners may notice changes that develop gradually: reduced activity, weight gain without eating more, a dull or matted coat, or a cat that seeks warmth more than usual. Because thyroid hormone affects many systems, the signs can be subtle and easily attributed to other causes, particularly in older animals. Some cats show few outward changes and the condition is identified during investigation of something else. This page explores the patterns that may suggest low thyroid function, what is happening at a glandular and cellular level, how the condition is investigated through blood tests and clinical assessment, and the approaches available once a picture emerges. The aim is to help you understand what hypothyroidism in cats can look like and how it is typically approached.
Why this matters now
Hypothyroidism in cats tends to appear in middle-aged to older animals, most commonly between seven and twelve years of age. The majority of cases occur as a consequence of previous treatment for hyperthyroidism—either surgical removal of thyroid tissue, radioactive iodine therapy, or prolonged use of anti-thyroid medication. A smaller proportion of cases arise as primary thyroid gland failure, where the gland itself becomes underactive without prior intervention, though this remains uncommon in cats compared to dogs.
The condition typically develops gradually, over weeks to months, as thyroid hormone levels decline and the body's metabolic processes begin to adjust. In cats with iatrogenic hypothyroidism—caused by treatment—the timeline can vary depending on how much functional thyroid tissue remains. Some cats may show signs within weeks of intervention, whilst others remain stable for months or years before changes become apparent. The pace and degree of progression can differ considerably between individuals, and some cats remain only mildly affected throughout.
Signals & patterns
Early signals
Reduced activity or lethargy
A cat may spend more time resting, show less interest in play, or move more slowly around the house. This change can be subtle and may be mistaken for normal ageing or a shift in temperament.
Weight gain without increased appetite
The cat may gain weight steadily despite eating the same amount, or even less, than before. This reflects a slowing of the metabolic rate, meaning fewer calories are burned at rest.
Coat changes
The fur may become dull, dry, or develop mats more easily, particularly in longer-haired cats. Grooming behaviour may not change, but the coat quality itself can deteriorate as skin and hair follicle function slows.
Increased heat-seeking behaviour
A cat may seek out warm places more than usual—sitting near radiators, on heated surfaces, or burrowing under blankets. This can reflect a reduced ability to generate body heat efficiently.
Later signals
Marked weight gain or obesity
In more advanced cases, weight gain can become pronounced, and the cat may develop a rounded or thickened appearance, particularly around the trunk. This occurs as metabolic slowing continues and energy expenditure remains low.
Mental dullness or reduced responsiveness
Some cats may appear less alert, take longer to respond to stimuli, or seem less engaged with their surroundings. This reflects the broader effects of low thyroid hormone on neurological function and overall cellular activity.
Persistent low body temperature
In some cases, body temperature may run consistently lower than normal, though this is typically identified during clinical examination rather than by owners at home.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
Last reviewed: Invalid Date ·