Tentative Diagnosis: The tentative diagnosis is primary hypothyroidism. This condition, often called the "great imitator," fits the case perfectly. The combination of signalment (middle-aged Golden Retriever), classic metabolic signs (lethargy, mental dullness, weight gain), hallmark dermatologic signs (symmetrical non-pruritic alopecia, rat tail), and key laboratory abnormalities (hypercholesterolemia, hypertriglyceridemia, mild non-regenerative anemia) makes hypothyroidism the leading differential.
Further Diagnostic Test(s):
While the clinical picture is highly suggestive, a definitive diagnosis requires a full thyroid panel.
Definitive Thyroid Panel: A blood sample was submitted for a complete thyroid profile.
Protocol: The panel measured total T4, free T4 by equilibrium dialysis (the most accurate method), and canine thyroid-stimulating hormone (cTSH).
Results:
Total T4 (TT4): 0.5 µg/dL (Markedly Low) [Ref: 1.0 - 4.0 µg/dL]
Free T4 by Equilibrium Dialysis (fT4 by ED): 4 pmol/L (Low) [Ref: 8 - 40 pmol/L]
Canine Thyroid Stimulating Hormone (cTSH): 1.5 ng/mL (Markedly High) [Ref: 0.05 - 0.6 ng/mL]
Interpretation: This is the classic and definitive diagnostic signature for primary hypothyroidism. The low TT4 and fT4 levels confirm that the thyroid gland is failing to produce hormones. The high cTSH level confirms that the pituitary gland is functioning correctly and is trying to stimulate a thyroid gland that is no longer capable of responding.
Definitive Diagnosis:
Primary Hypothyroidism. More information can be found at the
Treatment Plan: Treatment is straightforward and typically very rewarding, involving lifelong hormone supplementation.
Medication: Sadie was started on synthetic levothyroxine (L-thyroxine).
Dosing: The standard starting dose of 0.02 mg/kg was calculated, and she was prescribed 0.8 mg orally every 12 hours (BID). The owner was instructed to give the medication on a relatively empty stomach to ensure consistent absorption.
Expected Response: The owner was counseled on what to expect. An improvement in mental alertness and energy level should be noticeable within 1 to 3 weeks. The weight gain should stop, and she should begin to lose weight with appropriate diet and increased activity. The dermatologic signs are the slowest to resolve; hair regrowth may not be evident for 4 to 6 months.
Monitoring Protocol:
4-Week Recheck: Sadie is scheduled to return in one month for a therapeutic monitoring check.
Post-Pill T4: On the morning of her recheck, the owner will give the morning dose of levothyroxine as usual. The blood sample to measure her total T4 level will be drawn 4 to 6 hours after the pill is administered.
Goal: The goal is for the post-pill TT4 level to be in the high end of the normal reference range or slightly above it (approximately 2.5-4.5 µg/dL). The dose will be adjusted based on this result and Sadie's clinical response. This monitoring will be repeated periodically throughout her life.
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