Low T3 Syndrome is where low levels of T3 are present but other thyroid markers are normal. T3 is the most active form of thyroid so there will be physiological consequences. People experience many symptoms such as low body temperature, insomnia, dry skin, weight gain and fatigue. The pattern is also called euthyroid sick syndrome or non-thyroidal illness syndrome. Our thyroid gland doesn’t produce T3 so when T3 is low we have to look elsewhere.
How Thyroid Hormone Works
To understand the causes we must break-down the metabolism of thyroid hormone starting with the hypothalamus. Keep in mind that T3 is five times more biologically active than T4.
- The hypothalamus produces thyrotropin releasing hormone (TRH) when needed.
- TRH acts on the anterior pituitary to release thyroid stimulating hormone (TSH).
- TSH acts on the thyroid gland, which produces thyroxine (T4) and triiodothyronine (T3) in a ratio of 17:1.
- T4 is converted into T3 in multiple tissues and organs including the liver, gut, skeletal muscle, brain and the thyroid gland.
- Transport proteins in the liver (ex. thyroid binding globulin) carry T4 and T3 to the tissues, where they become free T4 and free T3.
- Free T4 and free T3 then bind to thyroid hormone receptors (THRs) and exert their metabolic effect.
Hypothyroidism is an issue at step 3. Low T3 can occur at any of the other steps.
Mechanisms of Low T3
- Stress affecting the HPA-Axis can result in T3 not reaching the tissue in adequate amount. In fact most studies on low T3 has been done on those in acute, life-threatening states of illness. More than 70% of patients in intensive care have low T3 and 50% have low T4.
- Carrier proteins not binding with the thyroid hormone.
- Conversion problem often caused by inflammation.
- Thyroid hormone not entering into the tissue.
- Modified expression or function of the thyroid hormone receptors.
Acute vs Chronic Illness
The lower the T3 level in critically ill patients, the worse the outcome tends to be. In non-critical, chronic illness more research is needed. The body converts excess T4 to reverse T3 (rT3) to conserve energy for healing and repair. It is possible that replacing thyroid hormone in these cases may not be beneficial.