# TSH-Secreting Adenoma and Thyroid Hormone Resistance

> TSH-secreting adenoma and thyroid hormone resistance for NEET-PG Medicine covers raised thyroxine with unsuppressed TSH and the tests that separate them.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/tsh-secreting-adenoma-and-thyroid-hormone-resistance
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Williams Textbook of Endocrinology; Harrison's Principles of Internal Medicine
- How to cite: "TSH-Secreting Adenoma and Thyroid Hormone Resistance", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/tsh-secreting-adenoma-and-thyroid-hormone-resistance

## Direct answer

A TSH-secreting adenoma is a rare pituitary tumour that secretes TSH autonomously, so thyroid hormones stay high while TSH is inappropriately normal or raised. Thyroid hormone resistance, usually from THRB gene mutations, produces a similar biochemical pattern because peripheral tissues respond poorly to thyroid hormone. Raised alpha-subunit and pituitary imaging point to the adenoma, while resistance is recognised by its pattern and confirmed by gene testing when needed.

## What you must remember

- **Shared pattern:** raised free T4 and T3 with a TSH that is not suppressed, the hallmark of both conditions and the reason they must be separated.
- **Alpha-subunit:** a raised alpha-subunit, with an alpha to TSH molar ratio above 1, supports a pituitary source of TSH.
- **Imaging:** pituitary MRI may show a micro- or macroadenoma, but a small tumour can be missed and imaging does not exclude resistance by itself.
- **Resistance:** usually dominant THRB mutations, giving goitre, tachycardia and attention difficulties, with the pituitary relatively resistant to thyroid hormone.
- **Adenoma treatment:** transsphenoidal surgery is preferred, with somatostatin analogues lowering TSH and shrinking tumours when surgery is not possible.
- **Resistance treatment:** no curative therapy exists, so beta-blockers control symptoms and ablation is avoided.
- **Mimic:** assay interference, especially from heterophile antibodies, can create a spurious pattern and must be excluded first.

## Separating central hyperthyroidism from resistance

A 42-year-old woman has palpitations, weight loss and a free T4 well above the reference range, yet her TSH is still within normal limits. Both diagnoses fit this pattern, so the work-up has to be ordered carefully. The steps below move from artefact to source of hormone.

1. **Exclude interference:** Check for heterophile antibodies or biotin interference and repeat the tests on an alternative assay before any pituitary work-up.
2. **Measure the alpha-subunit:** Send alpha-subunit alongside TSH, because an alpha to TSH molar ratio above 1 supports a TSH-secreting adenoma.
3. **Image the pituitary:** MRI locates most adenomas, although a microadenoma can be very small and imaging does not exclude resistance alone.
4. **Assess the peripheral picture:** Resistance often shows a goitre, tachycardia out of proportion to the illness, and a family history of raised thyroid tests.
5. **Confirm with genetics:** THRB sequencing settles the diagnosis when the picture is unclear, and it is useful for family screening.
6. **Plan treatment:** Operate or use somatostatin analogues for an adenoma, and avoid radioiodine or surgery for resistance.

## Where the exam tries to catch you

The common errors come from misreading a normal TSH as a Graves picture or assuming a single test settles the diagnosis.

- **Trap:** treating raised thyroid hormones with a normal TSH as Graves disease and giving radioiodine — **Correct:** Graves suppresses TSH, so a normal TSH argues against it, and radioiodine does not correct tissue resistance.
- **Trap:** assuming a normal alpha-subunit excludes an adenoma — **Correct:** the test is supportive rather than perfect, so imaging and the clinical picture also count.
- **Trap:** skipping the assay check — **Correct:** heterophile antibodies can give spurious raised hormones, so repeat on another platform first.

## Frequently asked questions

### Which test helps distinguish a TSH-secreting adenoma from thyroid hormone resistance?

The alpha-subunit to TSH molar ratio helps distinguish a TSH-secreting adenoma from thyroid hormone resistance, with a ratio above 1 supporting an adenoma. Pituitary MRI and THRB gene testing add useful information. No single test is perfect, so the clinical picture should be weighed alongside them.

### Why is radioiodine inappropriate in thyroid hormone resistance?

Radioiodine is inappropriate in thyroid hormone resistance because the problem is reduced tissue response, not excess hormone production. Ablation does not correct the tissue resistance and risks needless hypothyroidism. Symptoms are better managed with beta-blockers and monitoring.

### What is the alpha-subunit and why is it measured in TSH-secreting adenoma?

The alpha-subunit is a glycoprotein common to TSH, LH, FSH and hCG, so a tumour secreting TSH also releases it. A raised level relative to TSH, expressed as a molar ratio above 1, supports a pituitary source. Measuring it helps separate a TSH-secreting adenoma from other causes of inappropriate TSH.

### Which mutation causes thyroid hormone resistance?

Most cases of thyroid hormone resistance are caused by mutations in the THRB gene, which encodes thyroid hormone receptor beta. Inheritance is usually autosomal dominant, and the mutant receptor interferes with normal receptor signalling. Genetic testing is used when the diagnosis remains uncertain.

### Why is TSH inappropriately normal in a TSH-secreting adenoma?

In a TSH-secreting adenoma, the pituitary secretes TSH autonomously and does not respond to the negative feedback of high thyroid hormone levels. TSH therefore stays normal or raised despite a high free T4. This discordant pattern is the key clue for the diagnosis.