Levothyroxine Not Working? Here's What Science Says About Adding T3.

Most people with hypothyroidism are treated with levothyroxine, a synthetic form of the thyroid hormone thyroxine (T4) and this is considered first-line treatment. The body normally converts T4 into the more active hormone triiodothyronine (T3).

​

Why combination therapy?

T4 has a half-life of approximately seven days, providing stable hormone levels, whereas T3 (liothyronine) has a much shorter half-life. Microgram for microgram, T3 is roughly three to four times more potent than T4, so even small doses can have a substantial and rapid effect. For both these reasons, T3 is usually prescribed in much smaller doses than T4 and divided into twice-daily doses to reduce sharp peaks and fluctuations in hormone levels. If liothyronine is prescribed, it is normally added to levothyroxine (combination therapy) rather than used alone: the longer-acting T4 provides a steady reserve of thyroid hormone, while the smaller T3 dose supplements the active hormone. This more closely resembles normal thyroid function and reduces the sharp peaks and falls that can occur with short-acting T3 alone. Liothyronine by itself is generally not recommended, except in unusual circumstances (confirmed allergy or intolerance to levothyroxine or its ingredients).

Some people, despite levothyroxine use, continue to experience tiredness, difficulty concentrating, low mood, weight problems or a general sense that they are not completely well - even when their thyroid-stimulating hormone (TSH) is within the normal range.

 

So, does adding T3 help in such situations?

Clinical trials have not consistently shown combination therapy to address the residual symptoms. Nevertheless, some individual patients report feeling better with a treatment containing T3 and prefer it to levothyroxine alone. Combination treatment is therefore not currently recommended routinely, but it may be reasonable to consider it in carefully selected patients under specialist supervision. The phrase ‘not routinely’ is important. It does not amount to a complete prohibition, but it means that liothyronine is not considered standard treatment for most patients.

 

Before considering T3, clinicians should:

• Confirm that the patient originally had clear biochemical evidence of overt hypothyroidism.

• Make sure the levothyroxine dose has been properly optimized (TSH in the 0.3–2.0 mU/L range). It may be acceptable to have a serum TSH below reference range (0.1–0.3 mU/L), but never fully suppressed in the long term.

• Allow sufficient time (6 months) to assess the response to optimised treatment.

• Rule out other possible causes of these symptoms (anaemia, vitamin deficiencies, sleep problems, OSA, menopause, depression, anxiety, coeliac disease, medication effects or another medical condition).

If the above criteria are satisfied, a supervised trial of levothyroxine plus liothyronine may be considered, after shared decision-making. Treatment should be reviewed after an agreed trial period (6 months) and continued only if it produces a meaningful improvement in symptoms.

 

What about Desiccated Thyroid Extract (DTE)?

DTE (such as Armour Thyroid) is made from dried pig thyroid glands (bovine and ovine preparations are also available) and contains T4 and T3 in a fixed animal-derived ratio that differs from normal human thyroid-hormone production. DTE was the first therapy for hypothyroidism, prior to the synthesis of levothyroxine and has never been regulated by the FDA (because the use of this product predates the current approval process). One grain of DTE (usually 60 mg, although ‘grain’ terminology may vary between products), typically contains 38 micrograms of T4 and 9 micrograms of T3. This gives a T4:T3 ratio of approximately 4:1. That is considerably richer in T3 than normal human thyroid secretion, which has a ratio closer to approximately 14:1. Because T3 is roughly three to four times more potent than T4, the 9 micrograms of T3 makes a substantial contribution to the tablet’s overall effect. ATA in its 2025 statement recognises that some patients choose DTE and supports continued access – provided safety issues are addressed. Its disadvantages include short-lived T3 peaks, inability to adjust T4 and T3 independently, possible overtreatment, uncertain long-term safety, manufacturing and regulatory concerns, and unsuitability during pregnancy.

 

Is liothyronine safe?

When liothyronine was prescribed at medically recommended doses and appropriately monitored, there was no statistically significant increase in serious adverse events, atrial fibrillation, heart failure, stroke or death. Serious cases of toxicity were mainly associated with excessive doses, compounding errors, unregulated products bought online or use for weight loss or bodybuilding.

Too much thyroid hormone can cause:

• Palpitations or a rapid heartbeat

• Tremor

• Sweating or heat intolerance

• Anxiety and difficulty sleeping

• Muscle weakness

• Loss of bone strength over time (osteopenia and osteoporosis)

• Abnormal heart rhythms (including atrial fibrillation)

​

​

​

​

​

​

​

​

Extra caution is necessary for people with cardiovascular disease, heart-rhythm problems or osteoporosis risk. The aim is meaningful clinical improvement without evidence of excessive thyroid-hormone replacement. Anyone considering liothyronine should discuss it with an endocrinologist or another clinician experienced in thyroid-hormone treatment.