Addison's and hypothyroidism together in dogs: what to treat first
5-year-old Poodle bitch with confirmed hypoadrenocorticism, already stabilised after the crisis. On initial bloodwork total T4 was low and TSH mildly raised. The owner asks whether levothyroxine should also be started.
What is most appropriate?
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A. Start Addison's treatment and reassess thyroid function weeks later, before deciding whether levothyroxine is needed
Correct. Cortisol deficiency can lower T4 and raise TSH, which often normalise with glucocorticoids. If concurrent hypothyroidism is confirmed, levothyroxine is added afterwards: giving it first speeds cortisol metabolism and may precipitate a crisis.
B. Start levothyroxine before glucocorticoids, because the thyroid regulates the other glands
This is the dangerous order: levothyroxine increases cortisol clearance and metabolic demand, and can trigger an Addisonian crisis if the deficiency is not covered.
C. Start both treatments at full doses on the same day
Besides the risk of destabilising Addison's, it would treat hypothyroidism that may not exist: low T4 and high TSH may be a consequence of cortisol deficiency.
D. Dismiss any thyroid problem, because high TSH in a dog with Addison's is always an artefact
It is not always an artefact: autoimmune hypothyroidism can coexist (polyglandular syndrome). That is why it is reassessed once Addison's is treated.
lightbulb Key takeaway
With Addison's and thyroid abnormalities, treat the cortisol deficiency first and reassess the thyroid later. If levothyroxine is ultimately needed, introduce it once Addison's is controlled.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.