I'm a veterinarian from Finland, and I'd like your opinion and advice on a 2-year old, male neutered Doberman Pincher (body weight, 42 kg). This dog had been diagnosed with hypothyroidism together with serum T3 antibodies by another veterinarian.
Clinically, the dog has shown a partial response to treatment with L-thyroxine (L-T4), but serum T4 concentrations remain very low and serum TSH remains high on post-pill testing. These are results of my thyroid testing (always at 4 hours post-pill), together with the L-T4 dose and physical examination:
- Dose: Levothyroxine 400 µg BID
- Exam findings: Clinically quite normal but slightly overweight
- Total T4: <9.0 nmol/L (reference interval = 13 - 52 nmol/L)
- Free T4 (CLIA): >77 pmol/L (reference interval = 8 - 48 pmol/L )
- TSH: 3.24 ng/ml (reference interval = less than 0.5 ng/ml)
- Dose: Levothyroxine 500 µg BID
- Exam findings: Clinically normal; but still overweight
- Total T4: < 9.0 nmol/L
- TSH: 3.59 ng/ml
8-week recheck:I reviewed the L-T4 supplementation with the owner and made sure the dog is ingesting the pills and made certain that they administered the thyroid medication the morning of each recheck— no problem there. I next recommended repeat testing, again looking at the autoantibody levels and free T4 measured by equilibrium dialysis (ED).
- Dose: Levothyroxine 600 µg BID
- Exam findings: Mild weight loss, heart rate 82/bpm, very active and perky
- Total T4: <9.0 nmol/L
- Free T4 (CLIA): 62 pmol/L
- TSH: 2.2 ng/ml
- Dose: Levothyroxine 800 µg BID
- Exam findings: Normal
- Total T4: < 9.0 nmol/L
- Free T4 (ED): 8 pmol/L (reference interval = 6-40 pmol/L)
- TSH: 1.5 ng/ml
- T4 antibodies: 13% (1-20%)
- T3 antibodies: 43% (1-10%)
- Thyroglobulin autoantibodies: positive
I also need advice about how to determine the correct L-T4 replacement dose for this dog. Do I just keep on increasing the medication until the serum T4 concentration normalizes?
Obviously, your testing confirms that this dog has thyroiditis, with positive thyroglobulin autoantibodies and high levels of T3 autoantibodies (1-4). Based on the low serum concentrations of free T4 by dialysis and high TSH levels, the dog certainly is hypothyroid (5), but it does not appear that the dog is responding well to the L-T4 supplementation.
Method for T4 assay
What method is being used to measure total T4 concentrations? These days, most commercial veterinary labs don't use radioimmunoassay (RIA) to measure serum T4, even though that method is still considered the gold standard (5,6). Most laboratories use either a chemiluminescent immunoassay method (CLIA; Immulite) or an automated enzyme immunoassay (EIA) method (7,8).
For some reason not well understood, the EIA method will fail to accurately detect circulating post-pill T4 values, at least in some T4-treated dogs. In other words, the serum T4 values remain falsely low, even when the actual T4 values are within the mid- to high-normal range and the dogs are clinically improved. This issue needs more research and has not been published, but there is no doubt that the problem exists. If suspected, one can verify the problem very easily, simply by repeating the post-pill serum T4 concentration by either RIA or chemiluminescence.
Method for Free T4 assay
In this dog, the finding of a high free T4, measured by CLIA (Immulite) can not be easily explained. The fact that the serum TSH is persistently high in your dog suggests persistent hypothyroidism, not hyperthyroidism (5,9). The low-normal value for the free T4 concentration, when remeasured by equilibrium dialysis (ED), also points to hypothyroidism. Thyroid autoantibodies can not interfere with the free T4 value when determined by the dialysis method, since all protein molecules (including the thyroid autoantibodies) are removed (dialyzed) prior to assay.
Given that we know your dog has thyroid hormone autoantibodies, it's tempting to postulate that the high free T4 value measured by CLIA are falsely high due to the autoantibodies. The bottom line is that we just don't know. However, what is clear is that these free T4 values are not helping us to determine the best treatment for this dog — only confusing the situation.
Brand of L-thyroxine
What brand of LT4 is being administered? In any dog that is not responding well to thyroid hormone supplementation, a brand name product is always recommended. In addition, sometimes it can help to switch to another size or brand, suggesting that the absorption rate may vary slightly between products. In some studies, use of a liquid L-T4 solution has been found to be be better absorbed, at least in select patients (10,11).
Timing of L-T4 and meals
Are the owners giving the LT4 with meals or on an empty stomach? The standard of care for human patients requiring L-T4 is to administer the drug on an empty stomach, generally an hour before meals.
Simultaneous administration of L-T4 with food can markedly delay and inhibit the absorption of the drug in both humans and dogs (11-13). In one study of dogs, giving the L-T4 with food decreased its absorption by about 45% (11).
The serum T4 is being measured by EIA. We are giving a brand-name L-T4 product (14). The owner is consistently giving the L-T4 at 12-hour intervals at the time of meals. No other medication is being given.
Again, with the EIA method, the total T4 result may not be accurate when measuring a post-pill T4 concentration. We need to do the post-pill T4 by either chemiluminescence (CLIA; Immulite) or RIA on the next followup.
However, the fact that the free T4 concentration is low-normal and the TSH is still high means that the absorbed dose of L-T4 is not high enough. None of your problems appear to have anything to do with T4 or T3 autoantibodies.
I'd first change the timing between the L-T4 dose and feeding to ensure "empty stomach dosing." If that fails to adequately increase the total T4 concentration (RIA or CLIA) and normalize the high serum TSH concentrations, I'd next consider switching to a liquid L-T4 preparation to see if that helps. Finally, if all else fails, you may have to continue to increase the L-T4 dose, but at 0.8 mg twice daily, your dog is already on a more-than-adequate dose for most dogs (5).
- Refsal KR, Nachreiner RF. Thyroid hormone autoantibodies in the dog: their association with serum concentrations of iodothyronines and thyrotropin and distribution by age, sex, and breed of dog. Canine Practice 1997;22:16-17.
- Young DW. Antibodies to thyroid hormone and thyroglobulin in canine autoimmune lymphocytic thyroiditis. Canine Practice 1997;22:14-15.
- Nachreiner RF, Refsal KR, Graham PA, et al. Prevalence of serum thyroid hormone autoantibodies in dogs with clinical signs of hypothyroidism. J Am Vet Med Assoc 2002;220:466-471.
- Patzl M, Mostl E. Determination of autoantibodies to thyroglobulin, thyroxine and triiodothyronine in canine serum. J Vet Med A Physiol Pathol Clin Med 2003;50:72-78.
- Mooney CT, Shiel RE. Canine hypothyroidism In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;63-85.
- Kemppainen RJ, Birchfield JR. Measurement of total thyroxine concentration in serum from dogs and cats by use of various methods. Am J Vet Res 2006;67:259-265.
- Singh AK, Jiang Y, White T, et al. Validation of nonradioactive chemiluminescent immunoassay methods for the analysis of thyroxine and cortisol in blood samples obtained from dogs, cats, and horses. J Vet Diagn Invest 1997;9:261-268.
- Horney BS, MacKenzie AL, Burton SA, et al. Evaluation of an automated, homogeneous enzyme immunoassay for serum thyroxine measurement in dog and cat serum. Vet Clin Pathol 1999;28:20-28.
- Dixon RM, Reid SW, Mooney CT. Treatment and therapeutic monitoring of canine hypothyroidism. J Small Anim Pract 2002;43:334-340.
- Pirola I, Formenti AM, Gandossi E, et al. Oral liquid L-thyroxine (L-T4) may be better absorbed compared to L-T4 tablets following bariatric surgery. Obes Surg 2013;23:1493-1496.
- Le Traon G, Burgaud S, Horspool LJ. Pharmacokinetics of total thyroxine in dogs after administration of an oral solution of levothyroxine sodium. J Vet Pharmacol Ther 2008;31:95-101.
- Liwanpo L, Hershman JM. Conditions and drugs interfering with thyroxine absorption. Best Pract Res Clin Endocrinol Metab 2009;23:781-792.
- Lamson MJ, Pamplin CL, Rolleri RL, et al. Quantitation of a substantial reduction in levothyroxine (T4) absorption by food. Thyroid 2004;14:876.
- Forthyron LT4 product insert: http://www.forthyron.com/data/acms/docs/treatment/1_forthyron_pi_200_400_en.pdf