Feline hyperthyroidism is the most common hormone disease in cats over 10 years old, and a treated cat does not stay stable on its own. A cat that tests well in spring can drift out of range by late summer while still eating normally, and the first visible change is often 300 to 500 grams of muscle gone along the spine. The rhythm that works is a full panel every 3 months, 4 indicators kept on one sheet, and 2 classes of upward trend that trigger a phone call instead of a wait. Every dose decision stays with your veterinarian.
Why the Interval Is 3 Months
Total T4 has a short half-life of roughly 1 to 2 days, so any change in treatment shows up in blood within 7 to 14 days. That is why the first recheck after starting or adjusting treatment lands at 2 to 3 weeks, with a second look at 6 to 8 weeks if the value sits near the top of the range. Once 2 panels in a row fall inside the reference range, the interval stretches to 12 weeks, which is 3 months, for the rest of the cat's life. Stretching to 6 months is where most treated cats slip: kidney values and blood pressure move quietly between visits, and a cat can lose 10 percent of body weight before anyone notices. A cat with kidney disease, heart disease or a recent dose change stays on the 3-month cycle even when everything looks perfect. Cats treated with radioiodine instead of a daily tablet follow a different curve, and they need their own post-treatment recheck schedule rather than this one.
The 4 Indicators Worth Tracking
Four numbers cover most of what changes in a treated cat. Total T4 shows control. Creatinine plus SDMA show kidney function, which matters because treating hyperthyroidism can unmask hidden chronic kidney disease (CKD): a creatinine of 1.6 mg/dL on treatment may have been 1.2 mg/dL before the thyroid was controlled. Systolic blood pressure catches the hypertension that roughly 1 in 5 untreated hyperthyroid cats carries. Weight plus body condition score (BCS, 1 to 9) catches the muscle loss a scale hides for months. A fifth value, packed cell volume, is worth adding once a year.
| Indicator | Recheck interval | Watch line | Home data |
|---|---|---|---|
| Total T4 (TT4) | every 12 weeks | above lab upper limit, often 4.0 ug/dL | none |
| Creatinine + SDMA | every 12 weeks | creatinine up 0.3 mg/dL over baseline; SDMA over 14 ug/dL | none |
| Systolic blood pressure | every 12 weeks | 160 mmHg or higher | none |
| Weight + BCS | every 2 weeks at home | 5 percent loss in 8 weeks | grams, same scale |
| Sleeping respiratory rate | 3 times a week at home | 40 breaths per minute or higher | count 30 seconds, times 2 |
Trigger Class 1: T4 Climbing Back Over the Range
The first class of upward line is a T4 that returns above the lab's upper limit, commonly 4.0 ug/dL, though each laboratory prints its own ceiling. One high value is a data point, not an emergency: a cat that reads 4.6 ug/dL at week 12 with a normal appetite, stable weight and a sleeping respiratory rate under 30 breaths per minute can be rechecked in 2 to 4 weeks. Two high values in a row, or any single value above 7.5 ug/dL, means the plan gets reviewed before the next scheduled visit. Do not split or add tablets on your own to chase a number; call, read out the values and the dates, and let the vet set the dose.
Trigger Class 2: Kidney Values and Pressure Creeping Up
The second class is the one owners miss, because the cat still looks fine. A creatinine rise of 0.3 mg/dL or more above the cat's own baseline, an SDMA over 14 ug/dL, or a systolic reading of 160 mmHg or higher on 2 separate visits all point the same way: the current plan is now stressing the kidney or the heart. Common practice is to recheck those 3 values 2 to 4 weeks after any T4-driven change, then fold them back into the 3-month cycle. A cat at 180 mmHg or higher needs a retinal exam the same week, since detachment can happen within days and is irreversible.
What to Record Between Visits
Home data makes the 12-week visit 10 minutes shorter and far more useful, and on a tablet plan the log matters as much as the panel, which is what medication monitoring is for. Weigh the cat on the same scale every 2 weeks and write the grams down. Measure water intake over 24 hours once a month. Count the sleeping respiratory rate 3 times a week: under 30 breaths per minute is the line most cardiologists use for a cat at rest. Note vomiting, diarrhea, scratching around the head and neck, and any new reluctance to jump onto a 40 cm surface. Four lines per entry, 2 minutes of work, and the vet receives a trend instead of a snapshot.
Read the Trend, Not the Single Number
A single T4 of 3.9 ug/dL means little by itself. Three readings of 2.1, 3.2 and 3.9 ug/dL across 24 weeks describe a cat heading out of control. Plot them. A rise of 1.0 ug/dL between 2 consecutive panels deserves a call even when both values sit inside the printed range. The same logic applies to weight: 4.2 kg, then 3.9 kg, then 3.6 kg across 3 visits is a 14 percent loss, and the answer is to look at teeth, gut and T4 together rather than at the scale alone.
When to Call the Clinic the Same Day
Call today for a resting respiratory rate above 40 breaths per minute, open-mouth breathing, a cat that refuses food for 24 hours, vomiting more than 3 times in 12 hours, or a systolic reading of 200 mmHg or higher. Facial swelling, a head tilt, or sudden loss of vision after a treatment change goes on the urgent list too. A stable cat at 5.1 ug/dL with a normal appetite can wait for a scheduled slot; a cat that cannot breathe cannot wait at all.
A 5-Step Routine That Fits a Calendar
- Book the next 12-week panel before leaving the clinic, and put the date on the same calendar as the weighing days.
- Keep one sheet per cat with 4 columns: date, T4, creatinine and SDMA, systolic pressure, plus weight in grams.
- Weigh every 2 weeks on the same scale, and photograph the display so the number is never remembered wrongly.
- Write the 2 trigger classes on the sheet in plain words, so anyone in the house knows when to call.
- Take the sheet and 30 days of home notes to every visit, and ask for the lab's own reference range to be printed on it.
- Set a yearly budget from the cost table: 4 panels plus 1 blood pressure series is the floor, not the ceiling.
Terms mentioned in this article
Frequently asked
Can I stretch the recheck to every 6 months if my cat seems fine?
Most veterinarians keep treated cats on a 12-week cycle because T4, creatinine and blood pressure can all move well outside the reference range while appetite and behavior still look normal. A 6-month gap doubles the window in which kidney disease or hypertension can develop unnoticed, and both are common in cats over 12 years old. If two consecutive panels are inside range and the cat is stable, ask whether a 16-week interval is acceptable, but let the vet make that call.
My cat's T4 came back at 4.6 ug/dL. Is that an emergency?
Usually not. A single value slightly above the lab ceiling, in a cat that is eating, holding weight and breathing under 30 times a minute at rest, is normally rechecked in 2 to 4 weeks before any plan change. What raises urgency is two high values in a row, a reading above 7.5 ug/dL, or any of the same-day signs such as open-mouth breathing, refusing food for 24 hours, or a systolic pressure of 200 mmHg or more.
Do I need to do anything at home between the 3-month visits?
Four things take about 2 minutes each week: weigh the cat on the same scale every 2 weeks and write the grams down, count the sleeping respiratory rate 3 times a week, measure water intake over 24 hours once a month, and note any vomiting or new reluctance to jump. Written trends let your vet judge whether a value is drifting before it crosses a threshold, which is the whole point of a monitoring schedule.