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Thyforon (formerly called Forthyron) for Dogs

  • 200ug Tablet » Priced per Tablet £0.11
  • 400ug Tablet » Priced per Tablet £0.14
  • 800ug Tablet » Priced per Tablet £0.22

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Thyforon is the new form of the well established tablet formerly called Forthyron.  They are used to treat an under-active thyroid gland in dogs (known medically as hypothyroidism). This condition is common in middle aged dogs. It is not a life threatening condition, but it is often associated with a poor coat  and skin problems. It also tends to make affected individuals lethargic and overweight.  Reproductive problems are often seen in breeding animals too.Treatment of hypothyroidism in dogs is simple and safe using Thyforon or Forthyron. Both are tablets containing levothyroxine sodium. This is a synthetic product which is chemically identical to the naturally occurring hormone thyroxine, or “T4”. It is used to supplement deficient hormone levels in the affected dog and is to be used long term. Side effects are very rare with Thyforon/Forthyron unless it is significantly overdosed. Since it is chemically identical to the natural hormone, side effects only tend to occur due to increased effect. If a previously lethargic dog becomes more active due to improved thyroid hormone levels,  this can put a strain on the body if there are underlying heart or arthritic problems. Therefore it is sensible for your vet to assess the overall condition of your dog, including any other possible hormonal problems, before starting therapy. Thyforon tablets are different from Forthyron tablets is that they are flavoured to make them palatable to dogs, with most dogs taking them happily as treats

Treatment usually has to be given for several weeks before improvement in the skin and coat are seen. Dogs become more lively and alert usually more quickly.

Qualitative and quantitative composition

Each Thyforon flavoured 200 microgram tablet contains:

Active substance:

Levothyroxine sodium 200 microgram

(equivalent to 194 microgram levothyroxine)

Each Thyforon flavoured 400 microgram tablet contains:

Active substance:

Levothyroxine sodium 400 microgram

(equivalent to 389 microgram levothyroxine)

Each Thyforon flavoured 800 microgram tablet contains:

Active substance:

Levothyroxine sodium 800 microgram

(equivalent to 778 microgram levothyroxine)

For a full list of excipients, see Excipients.

Pharmaceutical form

Tablet. Off-white round tablet with brown spots, quadrisect with side scores. The tablets may be divided into halves or quarters.

Clinical particulars

Target species


Indications for use

For the treatment of hypothyroidism in dogs.


Do not use in dogs suffering from uncorrected adrenal insufficiency.

Do not use in cases of known hypersensitivity to levothyroxine sodium.

Special warnings for each target species

The diagnosis of hypothyroidism should be confirmed with appropriate tests.

Special precautions for use in animals

A sudden increase in demand for oxygen delivery to peripheral tissues, plus the chronotropic effects of levothyroxine sodium, may place undue stress on a poorly functioning heart, causing decompensation and signs of congestive heart failure.

Hypothyroid dogs suffering from hypoadrenocorticism have a decreased ability to metabolise levothyroxine sodium and therefore an increased risk of thyrotoxicosis. Dogs with concurrent hypoadrenocorticism and hypothyroidism should be stabilised with glucocorticoid and mineralocorticoid treatment prior to treatment with levothyroxine sodium to avoid precipitating a hypoadrenocortical crisis. After this, thyroid tests should be repeated, then gradual introduction of levothyroxine therapy, starting with 25% of the normal dose, increasing by 25% increments every fortnight until optimal stabilisation is achieved, is recommended. Gradual introduction of therapy is also recommended for dogs with other concurrent illnesses; particularly in dogs with cardiac disease, diabetes mellitus and renal or hepatic dysfunction.

Special precautions to be taken by the person administering the veterinary medicinal product to animals

Wash hands after administering the tablets. Pregnant women should handle the product with caution. In the case of accidental ingestion, seek medical advice immediately and show the package leaflet or the label to the physician. Note: this product contains a high concentration of L-thyroxine sodium and may present a risk to humans, in particular children, if ingested.

Adverse reactions

Restoration of physical activity may unmask or intensify other problems, such as osteoarthrosis.

Adverse reactions of thyroid hormones are generally associated with excessive dosage and correspond to the symptoms of hyperthyroidism.

See also Overdose.

Use during pregnancy and lactation

The safety of the veterinary medicinal product has not been established in pregnant or lactating bitches. However, levothyroxine is an endogenous substance and thyroid hormones are essential for the developing foetus, especially during the first period of gestation. Hypothyroidism during pregnancy may result in major complications such as foetal death and a poor perinatal outcome. Maintenance doses of levothyroxine sodium may need adjustment during pregnancy. Pregnant bitches should therefore be monitored on a regular basis from conception until several weeks after delivery.


A variety of drugs may impair plasma or tissue binding of the thyroid hormones or alter thyroid hormone metabolism (e.g. barbiturates, antacids, anabolic steroids, diazepam, furosemide, mitotane, phenylbutazone, phenytoin, propranolol, large doses of salicylates and sulphonamides). When treating dogs that are receiving concurrent medication the properties of these drugs should be taken into consideration.

Oestrogens may increase thyroid requirements.

Ketamine may cause tachycardia and hypertension when used in patients receiving thyroid hormones.

The effect of catecholamines and sympaticomimetics is increased by levothyroxine.

An increase in the dosage of digitalis may be necessary in a patient that had previously compensated congestive heart failure and that is placed on thyroid hormone supplementation. Following treatment of hypothyroidism in dogs with concurrent diabetes, careful monitoring of diabetic control is recommended.

Most dogs on chronic high dose, daily glucocorticoid therapy will have very low or undetectable serum T4 concentrations, as well as subnormal T3 values.

Amounts to be administered and administration route

The recommended starting dosage of levothyroxine sodium is 10 µg/kg body weight orally every 12 hours. Because of variability in absorption and metabolism, the dosage may require alterations before a complete clinical response is observed. The initial dosage and frequency of administration are merely a starting point. Therapy has to be highly individualised and tailored to the requirements of the individual dog. When initiating dosing of dogs weighing less than 5 kg body weight, a quarter of one 200 μg tablet should be administered once daily. Such cases should be monitored carefully. In the dog, absorption of levothyroxine sodium may be affected by the presence of food. The timing of treatment and its relation to feeding should therefore be kept consistent from day to day. To adequately monitor therapy, trough values (just prior to treatment) and peak values (about three hours after dosing) of plasma T4 can be measured. In adequately dosed dogs peak plasma concentration of T4 should be in the high-normal range (approximately 30 to 47 nmol/l) and trough values should be above approximately 19 nmol/l. If T4 levels are outside this range the levothyroxine dose can be adjusted in 50 to 200 µg increments until the patient is clinically euthyroid and serum T4 is within the reference range. Plasma T4 levels can be retested two weeks after change of dosage, but clinical improvement is an equally important factor in determining individual dosage and this will take four to eight weeks. When the optimum replacement dose has been attained, clinical and biochemical monitoring may be performed every 6-12 months.

To break a tablet accurately and easily, place the tablet score side up and apply pressure with your thumb.

To break the tablet in two parts; hold one half of the tablet down and press down the other half.


Following administration of overdoses thyrotoxicosis could occur. Thyrotoxicosis as a side effect of mild over-supplementation is uncommon in dogs, owing to the canine ability to catabolise and excrete thyroid hormones. In case of accidental intake of large amounts of the veterinary medicinal product absorption can be decreased by induction of vomiting and oral administration of both activated charcoal and magnesium sulphate once.

Overdoses of three up to six times label recommended starting dose for 4 consecutive weeks in healthy, euthyroid dogs resulted in no significant clinical signs that could be attributed to treatment. Single overdose up to 3-6 x the recommended dose does not pose a threat to the dog, and no actions are necessary. However, following chronic over-supplementation, clinical signs of hyperthyroidism such as polydipsia, polyuria, panting, weight loss without anorexia, and either or both tachycardia and nervousness may theoretically occur. The presence of these signs should result in evaluation of T4 serum concentrations to confirm the diagnosis, and immediate discontinuance of the supplementation. Once the signs have abated (days to weeks), the thyroid dosage has been reviewed, and the animal has fully recovered, a lower dosage may be instituted, with the animal being monitored closely.

Pharmacological particulars

Pharmacotherapeutic group: Synthetic thyroid hormones.

ATC Vet code: QH03AA01

Pharmacodynamic properties

Pharmacologically levothyroxine is classified as a hormonal preparation that replaces deficient endogenous hormones.

Levothyroxine T4 is converted to triiodothyronine T3. T3 acts on cellular processes via specific ligand-receptor interactions with the nucleus, the mitochondria, and the plasma membrane. Interaction of T3 with binding sites leads to augmented transcription of DNA or modulation of RNA, thus influencing protein synthesis and enzyme action.

Thyroid hormones act on many different cellular processes. In developing animals and human beings, they are crucial determinants of normal development, especially in the central nervous system. Thyroid supplementation increases basal cellular metabolism and oxygen consumption thereby affecting the function of virtually all organ systems.

Pharmacokinetic properties

Some dogs appeared to consistently either absorb L-thyroxine better and/or eliminate it more slowly than did other dogs. Furthermore, absorption and elimination rate is influenced by daily intake of levothyroxine sodium (high absorption/low elimination in case of low intake and vice versa in case of high intake). The variability in pharmacokinetic parameters between individual dogs is considerable and, although the presence of food may affect absorption, it is considered to have a minor effect on the parameters overall.

Absorption is relatively slow and incomplete: In most cases Tmax occurs between 1 to 5 hours after oral administration, mean Cmax varies more than 3 fold between dogs on the same doses. In adequately dosed dogs the plasma peak approaches or slightly exceeds the upper limit of normal plasma T4 levels, and by the end of 12 hours after oral administration, plasma T4 usually declines to the lower half of the normal range. The rates of disappearance of T4 from the plasma are slowed in hypothyroidism. A large part of the thyroxine is taken up by the liver. L-thyroxine is bound to plasma-proteins and plasma lipoproteins. Part of a dose of thyroxine is metabolised to the more potent triiodothyronine (T3) by deiodination. The process of deiodination continues. These further deiodinated metabolic products (other than T3 and T4) do not have thyromimetic activity. Other pathways of thyroid hormone metabolism include conjugation to form soluble glucuronides and sulphates for biliary or urinary excretion as well as cleavage of the ether linkage of the iodothyronine molecule. In the dog, over 50% of the T4 produced each day are lost in the faeces. The extrathyroidal body stores of T4 are eliminated and replaced in about 1 day.

Pharmaceutical particulars


Calcium hydrogen phosphate dihydrate, cellulose (microrcrystalline), sodium starch glycolate (type A), magnesium stearate, natural meat flavour.

Shelf life

Shelf life of the veterinary medicinal product as packaged for sale: 2 years.

Shelf life of remaining tablet parts: 4 days.

Do not use after the expiry date stated on the carton and the blister after EXP.

Special precautions for storage

Do not store above 25°C. Return any divided tablet to the opened blister and use within 4 days. Keep out of the reach and sight of children.

Immediate packaging

The product is packaged in a blister [Aluminium (20 µm) - PVC/PE/PVDC (250/30/90) white]. 10 Tablets per blister, 5 or 25 blisters per carton, 50 or 250 tablets per carton. Not all pack sizes may be marketed.


Any unused veterinary medicinal product or waste materials derived from such veterinary medicinal products should be disposed of in accordance with local requirements.

Marketing authorisation holder (if different from distributor)

Eurovet Animal Health BV, Handelsweg 25, 5531 AE Bladel, The Netherlands.

Marketing authorisation number

Thyforon flavoured 200 microgram tablet for dogs: Vm 16849/4034

Thyforon flavoured 400 microgram tablet for dogs: Vm 16849/4035

Thyforon flavoured 800 microgram tablet for dogs: Vm 16849/4037.

Date of the first authorisation or date of renewal


Date of revision of the text

November 2011

Any other information

For animal treatment only. To be supplied only on veterinary prescription.

Manufacturing authorisation holder responsible for batch release: Eurovet Animal Health BV, Handelsweg 25, 5531 AE Bladel, The Netherlands.

Legal category


GTIN (Global Trade Item No)

Thyforon Flavoured Tablets for Dogs 200 microgram 250 tablets:


Thyforon Flavoured Tablets for Dogs 400 microgram 250 tablets:


Thyforon Flavoured Tablets for Dogs 800 microgram 250 tablets:


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Reviews of Thyforon (formerly called Forthyron) for Dogs

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Questions & Answers for Thyforon (formerly called Forthyron) for Dogs

Below are some recent questions we've received regarding Thyforon (formerly called Forthyron) for Dogs, including answers from our team.

Ask Your Own Question

About Thyroxine

15th Jun 2015
Ivan Cheung
  • VioVet Customer Since: June 2015
  • From: , Hong Kong

HI My senior doggie must need to had
"Thyroxine (ThyroMed)0.2mg twice time per day,is't same with this product?

John Cousins
  • Veterinary Surgeon

There are several different brands of thyroid hormone tablets for use in dogs. All of them can be used once a day or twice a day. Some dogs do appear to respond better to twice daily dosing and some vets always advise this. Other vets will try once per day at first. The active ingredient is the same in all the top brands, so you should aim to give a new brand in the same way as you gave the previous brand, while always checking with your own vet first. You should probably expect to give this product twice per day to your dog.

Delivery to Germany

5th Jan 2014

hello there,

do you deliver "Typhoron" (prescription) to Germany or only within U.K.?

sincerely yours


John Cousins
  • Veterinary Surgeon

If you send us a veterinary prescription to cover the medication you order, then we can send medication to Germany.