Thyroid function tests - Thyroid hormone resistance

Beatrice Ranasinghe; Navpreet Chhina

doi:10.1530/endoabs.82.WC2

Case history: 61 year old female with a history of inherited dilated cardiomyopathy was referred with abnormal thyroid functions not improving with Levothyroxine. She has been on Levothyroxine 100 mg which she has discontinued 5 months prior to the review but her thyroid function abnormality persisted. She had no family history of thyroid abnormalities.

Investigations: Negative TSH receptor antibodies.Treatment and follow up:


	TSH (mUnit/l)	fT4 (pmol/l)	fT3 (pmol/l)
Aug 2019	3	35	-
Nov 2019	2.68	49	-
Jan 2020	5.22	41.6	-
March 2020	4.93	37.9	9.3
June 2020	11.8	29	11.2
August 2020	5.79	30.6	7.6
May 2021	6.47	32	8.4
Negative TSH receptor antibodies.

Above thyroid function results raised 2 main clinical suspicions which are Thyroid hormone resistance and a TSHoma. She was further investigated with a MRI pituitary which was normal.


	TSH (mUnit/l)	fT4 (pmol/l)	fT3 (pmol/l)
Aug 2019	3	35	-
Nov 2019	2.68	49	-
Jan 2020	5.22	41.6	-
March 2020	4.93	37.9	9.3
June 2020	11.8	29	11.2
August 2020	5.79	30.6	7.6
May 2021	6.47	32	8.4
Negative TSH receptor antibodies.

No assay interference confirmed on Delfia blood test at Cambridge lab.


Delfia assay		Centaur assay
TSH (mUnit/l)	3.2
fT4 (pmol/l)	31.9	26.5
Calculated fT4 (based on total T4 and TBG)	24
fT3 (pmol/l)	-	6.82
TBG	18.6

Genetic testing: Heterozygous for THRB-related thyroid hormone resistance

Conclusion and points for discussion: • In the presence of a raised fT4 with an unsuppressed TSH; a TSH secreting adenoma and thyroid hormone resistance should be the main differential diagnosis.

• There are two types of thyroid hormone resistance;

1. Generalised resistance to thyroid hormone (GRTH)

• Most present with a goitre or incidentally found abnormal thyroid functions

• Also, may present with mild hyperthyroidism, deaf mutism or delayed bone maturation

• Usually does not need treatment

2. Selective pituitary resistance to thyroid hormone (PRTH)

• Patients exhibit definite clinical manifestations of thyrotoxicosis

• Need a chronic suppression of TSH secretion with D T4, tri-iodothyroacetic acid, Octrotide or Bromocriptine.

• If medical management is ineffective, they would need thyroid ablation with radioiodine or surgery.

• 85% of thyroid hormone resistance results from gene encoding TRbeta, and its identification confirms the diagnosis. Normally the affected individuals will be heterozygous with an autosomal dominant inheritance pattern.

Endocrine Abstracts

WC2