Thyroid hormone binding ratio (THBR) is a
thyroid function test
Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid.
TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid) ...
that measures the "uptake" of
T3 or T4 tracer by
thyroid-binding globulin (TBG) in a given serum sample. This provides an indirect and ''reciprocal'' estimate of the available binding sites on TBG within the sample.
The results are then reported as a ratio to normal serum.
Indications
Attempts to correct for changes in thyroid binding globulin due to liver disease, protein losing states, pregnancy or various drugs
It is used to calculate
free thyroxine index (total T4 x T3 uptake), an estimate of free T4. Free thyroxine index may be calculated with increased diagnostic accuracy using direct TBG measurement when the total hormone concentration is abnormally elevated
Examples
* In patients with
hyperthyroidism
Hyperthyroidism is a endocrine disease in which the thyroid gland produces excessive amounts of thyroid hormones. Thyrotoxicosis is a condition that occurs due to elevated levels of thyroid hormones of any cause and therefore includes hyperth ...
, there will be fewer available binding sites on TBG (due to the increased circulating T3 / T4). This will lead to an increased thyroid hormone binding ratio.
* In patients with
hypothyroidism
Hypothyroidism is an endocrine disease in which the thyroid gland does not produce enough thyroid hormones. It can cause a number of symptoms, such as cold intolerance, poor ability to tolerate cold, fatigue, extreme fatigue, muscle aches, co ...
, there will be more free binding sites on TBG (due to the decreased amount of circulating T3 / T4) and as such the THBR will be decreased.
* In general, High with High thyroid activity and Low with Low thyroid activity.
Other Conditions
* Total TBG can be increased (thereby decreasing the THBR) congenitally, or in conditions such as pregnancy (period of increased
estrogen
Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three ...
) and with the treatment of certain infections such as Hepatitis C. In the latter, reduction of inflammation of the liver results in increased protein synthesis
* Total TBG can be decreased (thereby increasing the THBR) congenitally, or in conditions such as liver failure,
protein-losing conditions, or
nephrotic conditions. Increased
androgen
An androgen (from Greek ''andr-'', the stem of the word meaning ) is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes ...
levels will also decrease TBG synthesis, increasing THBR.
* THBR can be directly altered by drugs such as;
**
Anticonvulsant
Anticonvulsants (also known as antiepileptic drugs, antiseizure drugs, or anti-seizure medications (ASM)) are a diverse group of pharmacological agents used in the treatment of epileptic seizures. Anticonvulsants are also used in the treatme ...
s such as
phenytoin
Phenytoin (PHT), sold under the brand name Dilantin among others, is an anticonvulsant, anti-seizure medication. It is useful for the prevention of tonic-clonic seizures (also known as grand mal seizures) and focal seizures, but not absence se ...
and
carbamazepine
Carbamazepine, sold under the brand name Tegretol among others, is an anticonvulsant medication used in the treatment of epilepsy and neuropathic pain. It is used as an adjunctive treatment in schizophrenia along with other medications and as ...
** Antinflammatory drugs such as
salicylates
Salicylic acid is an organic compound with the formula HOC6H4COOH. A colorless (or white), bitter-tasting solid, it is a precursor to and a metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and has been listed by the EPA Tox ...
(Aspirin) or
phenylbutazone
Phenylbutazone, often referred to as "bute", is a nonsteroidal anti-inflammatory drug (NSAID) for the short-term treatment of pain and fever in animals.
In the United States and United Kingdom, it is no longer approved for human use (except in t ...
(NSAID)
** High levels of
free fatty acids
In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
, commonly seen in acutely ill patients.
Limitations
Invalid if other proteins or immunoglobulins compete with TBG, including
familial dysalbuminemic hyperthyroxinemia
References
{{reflist
Chemical pathology
Blood tests
Endocrine procedures
Thyroidological methods
Endocrine function tests